diff --git a/src/Buffer.cpp b/src/Buffer.cpp
index f7f12c8127..829616a205 100644
--- a/src/Buffer.cpp
+++ b/src/Buffer.cpp
@@ -1586,10 +1586,10 @@ bool Buffer::makeLaTeXFile(FileName const & fname,
 {
 	OutputParams runparams = runparams_in;
 
-	// This is necessary for LuaTeX/XeTeX with tex fonts.
-	// See FIXME in BufferParams::encoding()
-	if (runparams.isFullUnicode())
-		runparams.encoding = encodings.fromLyXName("utf8-plain");
+	// XeTeX with TeX fonts is only safe with ASCII encoding,
+	// See #9740 and FIXME in BufferParams::encoding()
+	if (params().useNonTeXFonts && (runparams.flavor == OutputParams::XETEX))
+		runparams.encoding = encodings.fromLyXName("ascii");
 
 	string const encoding = runparams.encoding->iconvName();
 	LYXERR(Debug::LATEX, "makeLaTeXFile encoding: " << encoding << ", fname=" << fname.realPath());
@@ -1673,10 +1673,10 @@ void Buffer::writeLaTeXSource(otexstream & os,
 
 	OutputParams runparams = runparams_in;
 
-	// This is necessary for LuaTeX/XeTeX with tex fonts.
-	// See FIXME in BufferParams::encoding()
-	if (runparams.isFullUnicode())
-		runparams.encoding = encodings.fromLyXName("utf8-plain");
+	// XeTeX with TeX fonts is only safe with ASCII encoding,
+	// See #9740 and FIXME in BufferParams::encoding()
+	if (params().useNonTeXFonts && (runparams.flavor == OutputParams::XETEX))
+		runparams.encoding = encodings.fromLyXName("ascii");
 
 	// If we are compiling a file standalone, even if this is the
 	// child of some other buffer, let's cut the link here, so the
diff --git a/src/BufferParams.cpp b/src/BufferParams.cpp
index dddbf78e4e..537178e2fa 100644
--- a/src/BufferParams.cpp
+++ b/src/BufferParams.cpp
@@ -2307,7 +2307,7 @@ string BufferParams::bufferFormat() const
 	string format = documentClass().outputFormat();
 	if (format == "latex") {
 		if (useNonTeXFonts)
-			return "xetex";
+			return "xetex"; // FIXME: why not "luatex"?
 		if (encoding().package() == Encoding::japanese)
 			return "platex";
 	}
@@ -2927,20 +2927,16 @@ docstring BufferParams::getGraphicsDriver(string const & package) const
 void BufferParams::writeEncodingPreamble(otexstream & os,
 					 LaTeXFeatures & features) const
 {
-	// XeTeX does not need this
+	// "inputenc" package not required with non-TeX fonts.
+	if (useNonTeXFonts)
+		return;
+	// "inputenc"  fails with XeTeX (even in 8-bit compatiblitly mode) and with TeX fonts,
+	// (this is a bug in the "inputenc" package see #9740).
 	if (features.runparams().flavor == OutputParams::XETEX)
 		return;
-	// LuaTeX neither, but with tex fonts, we need to load
-	// the luainputenc package.
-	if (features.runparams().flavor == OutputParams::LUATEX
-		|| features.runparams().flavor == OutputParams::DVILUATEX) {
-		if (!useNonTeXFonts && inputenc != "default"
-		    && ((inputenc == "auto" && language->encoding()->package() == Encoding::inputenc)
-		        || (inputenc != "auto" && encoding().package() == Encoding::inputenc))) {
-			os << "\\usepackage[utf8]{luainputenc}\n";
-		}
-		return;
-	}
+	// For LuaTeX with TeX fonts, we can load
+	// the "luainputenc" package with the specified encoding(s) (see below).
+
 	if (inputenc == "auto") {
 		string const doc_encoding =
 			language->encoding()->latexName();
@@ -2972,7 +2968,11 @@ void BufferParams::writeEncodingPreamble(otexstream & os,
 					os << ',';
 				os << from_ascii(doc_encoding);
 			}
-			os << "]{inputenc}\n";
+		   	if (features.runparams().flavor == OutputParams::LUATEX
+			    || features.runparams().flavor == OutputParams::DVILUATEX)
+				os << "]{luainputenc}\n";
+			else
+				os << "]{inputenc}\n";
 		}
 		if (package == Encoding::CJK || features.mustProvide("CJK")) {
 			if (language->encoding()->name() == "utf8-cjk"
@@ -2992,8 +2992,12 @@ void BufferParams::writeEncodingPreamble(otexstream & os,
 			if (features.isRequired("japanese")
 			    || features.isProvided("inputenc"))
 				break;
-			os << "\\usepackage[" << from_ascii(encoding().latexName())
-			   << "]{inputenc}\n";
+			os << "\\usepackage[" << from_ascii(encoding().latexName());
+		   	if (features.runparams().flavor == OutputParams::LUATEX
+			    || features.runparams().flavor == OutputParams::DVILUATEX)
+				os << "]{luainputenc}\n";
+			else
+				os << "]{inputenc}\n";
 			break;
 		case Encoding::CJK:
 			if (encoding().name() == "utf8-cjk"
@@ -3123,10 +3127,10 @@ string const BufferParams::loadFonts(LaTeXFeatures & features) const
 
 Encoding const & BufferParams::encoding() const
 {
-	// FIXME: actually, we should check for the flavor
-	// or runparams.isFullyUnicode() here:
-	// This check will not work with XeTeX/LuaTeX and tex fonts.
-	// Thus we have to reset the encoding in Buffer::makeLaTeXFile
+	// FIXME: additionally, we must check for runparams().flavor == XeTeX
+	// or runparams.isFullUnicode() to care for the combination
+	// of XeTeX and TeX-fonts (see #9740).
+	// Currently, we reset the encoding in Buffer::makeLaTeXFile
 	// (for export) and Buffer::writeLaTeXSource (for preview).
 	if (useNonTeXFonts)
 		return *(encodings.fromLyXName("utf8-plain"));
diff --git a/src/PDFOptions.cpp b/src/PDFOptions.cpp
index 408fa9cc9d..afbd5e5f64 100644
--- a/src/PDFOptions.cpp
+++ b/src/PDFOptions.cpp
@@ -176,7 +176,7 @@ void PDFOptions::writeLaTeX(OutputParams & runparams, otexstream & os,
 
 	// hyperref expects utf8!
 	if (need_unicode && enc && enc->iconvName() != "UTF-8"
-	    &&!runparams.isFullUnicode()) {
+	    &&!runparams.isFullUnicode()) { // FIXME: check must be done for useNonTeXFonts!
 		os << "\\inputencoding{utf8}\n"
 		   << setEncoding("UTF-8");
 	}
@@ -196,7 +196,7 @@ void PDFOptions::writeLaTeX(OutputParams & runparams, otexstream & os,
 		os << from_utf8(opt);
 
 	if (need_unicode && enc && enc->iconvName() != "UTF-8"
-	    &&!runparams.isFullUnicode()) {
+	    &&!runparams.isFullUnicode()) { // FIXME: check for useNonTeXFonts!
 		os << setEncoding(enc->iconvName())
 		   << "\\inputencoding{" << from_ascii(enc->latexName()) << "}\n";
 	}
diff --git a/src/Paragraph.cpp b/src/Paragraph.cpp
index 5cea164d63..60668124b6 100644
--- a/src/Paragraph.cpp
+++ b/src/Paragraph.cpp
@@ -2567,7 +2567,7 @@ void Paragraph::latex(BufferParams const & bparams,
 	if (allowcust && d->endTeXParParams(bparams, os, runparams)
 	    && runparams.encoding != prev_encoding) {
 		runparams.encoding = prev_encoding;
-		if (!runparams.isFullUnicode())
+		if (!runparams.isFullUnicode()) // FIXME: test for UseTeXFonts
 			os << setEncoding(prev_encoding->iconvName());
 	}
 
diff --git a/src/output_latex.cpp b/src/output_latex.cpp
index cca533bab4..7b633810d2 100644
--- a/src/output_latex.cpp
+++ b/src/output_latex.cpp
@@ -248,7 +248,7 @@ static void finishEnvironment(otexstream & os, OutputParams const & runparams,
 		state->prev_env_language_ = data.par_language;
 		if (runparams.encoding != data.prev_encoding) {
 			runparams.encoding = data.prev_encoding;
-			if (!runparams.isFullUnicode())
+			if (!runparams.isFullUnicode()) // FIXME: test for UseTeXFonts
 				os << setEncoding(data.prev_encoding->iconvName());
 		}
 	}
@@ -258,7 +258,7 @@ static void finishEnvironment(otexstream & os, OutputParams const & runparams,
 		state->prev_env_language_ = data.par_language;
 		if (runparams.encoding != data.prev_encoding) {
 			runparams.encoding = data.prev_encoding;
-			if (!runparams.isFullUnicode())
+			if (!runparams.isFullUnicode()) // FIXME: test for UseTeXFonts
 				os << setEncoding(data.prev_encoding->iconvName());
 		}
 	}
@@ -878,7 +878,7 @@ void TeXOnePar(Buffer const & buf,
 			latexArgInsets(par, os, runparams, style.postcommandargs(), "post:");
 		if (runparams.encoding != prev_encoding) {
 			runparams.encoding = prev_encoding;
-			if (!runparams.isFullUnicode())
+			if (!runparams.isFullUnicode()) // FIXME: test for UseTeXFonts
 				os << setEncoding(prev_encoding->iconvName());
 		}
 	}
@@ -1040,12 +1040,11 @@ void TeXOnePar(Buffer const & buf,
 	// If this is the last paragraph, and a local_font was set upon entering
 	// the inset, and we're using "auto" or "default" encoding, the encoding
 	// should be set back to that local_font's encoding.
-	// However, do not change the encoding when a fully unicode aware backend
-	// such as XeTeX is used.
+	// However, do not change the encoding when non-TeX fonts are used.
 	if (runparams.isLastPar && runparams_in.local_font != 0
 	    && runparams_in.encoding != runparams_in.local_font->language()->encoding()
 	    && (bparams.inputenc == "auto" || bparams.inputenc == "default")
-	    && (!runparams.isFullUnicode())) {
+	    && (!runparams.isFullUnicode())) { // FIXME: test for UseTeXFonts
 		runparams_in.encoding = runparams_in.local_font->language()->encoding();
 		os << setEncoding(runparams_in.encoding->iconvName());
 	}