diff --git a/examples/SpinBox.py b/examples/SpinBox.py
index 2faf10ee..ef1d0fc5 100644
--- a/examples/SpinBox.py
+++ b/examples/SpinBox.py
@@ -26,7 +26,7 @@ spins = [
     ("Float with SI-prefixed units<br>(n, u, m, k, M, etc)", 
      pg.SpinBox(value=0.9, suffix='V', siPrefix=True)),
     ("Float with SI-prefixed units,<br>dec step=0.1, minStep=0.1", 
-     pg.SpinBox(value=1.0, suffix='V', siPrefix=True, dec=True, step=0.1, minStep=0.1)),
+     pg.SpinBox(value=1.0, suffix='PSI', siPrefix=True, dec=True, step=0.1, minStep=0.1)),
     ("Float with SI-prefixed units,<br>dec step=0.5, minStep=0.01", 
      pg.SpinBox(value=1.0, suffix='V', siPrefix=True, dec=True, step=0.5, minStep=0.01)),
     ("Float with SI-prefixed units,<br>dec step=1.0, minStep=0.001", 
diff --git a/pyqtgraph/functions.py b/pyqtgraph/functions.py
index 7ad603f7..0495a00c 100644
--- a/pyqtgraph/functions.py
+++ b/pyqtgraph/functions.py
@@ -110,7 +110,7 @@ def siFormat(x, precision=3, suffix='', space=True, error=None, minVal=1e-25, al
         return fmt % (x*p, pref, suffix, plusminus, siFormat(error, precision=precision, suffix=suffix, space=space, minVal=minVal))
 
 
-def siParse(s, regex=FLOAT_REGEX):
+def siParse(s, regex=FLOAT_REGEX, suffix=None):
     """Convert a value written in SI notation to a tuple (number, si_prefix, suffix).
     
     Example::
@@ -118,6 +118,12 @@ def siParse(s, regex=FLOAT_REGEX):
         siParse('100 μV")  # returns ('100', 'μ', 'V')
     """
     s = asUnicode(s)
+    s = s.strip()
+    if suffix is not None and len(suffix) > 0:
+        if s[-len(suffix):] != suffix:
+            raise ValueError("String '%s' does not have the expected suffix '%s'" % (s, suffix))
+        s = s[:-len(suffix)] + 'X'  # add a fake suffix so the regex still picks up the si prefix
+        
     m = regex.match(s)
     if m is None:
         raise ValueError('Cannot parse number "%s"' % s)
@@ -126,15 +132,18 @@ def siParse(s, regex=FLOAT_REGEX):
     except IndexError:
         sip = ''
     
-    try:
-        suf = m.group('suffix')
-    except IndexError:
-        suf = ''
+    if suffix is None:
+        try:
+            suf = m.group('suffix')
+        except IndexError:
+            suf = ''
+    else:
+        suf = suffix
     
     return m.group('number'), '' if sip is None else sip, '' if suf is None else suf 
 
 
-def siEval(s, typ=float, regex=FLOAT_REGEX):
+def siEval(s, typ=float, regex=FLOAT_REGEX, suffix=None):
     """
     Convert a value written in SI notation to its equivalent prefixless value.
 
@@ -142,9 +151,9 @@ def siEval(s, typ=float, regex=FLOAT_REGEX):
     
         siEval("100 μV")  # returns 0.0001
     """
-    val, siprefix, suffix = siParse(s, regex)
+    val, siprefix, suffix = siParse(s, regex, suffix=suffix)
     v = typ(val)
-    return siApply(val, siprefix)
+    return siApply(v, siprefix)
 
     
 def siApply(val, siprefix):
diff --git a/pyqtgraph/widgets/SpinBox.py b/pyqtgraph/widgets/SpinBox.py
index b8066cd7..17caea32 100644
--- a/pyqtgraph/widgets/SpinBox.py
+++ b/pyqtgraph/widgets/SpinBox.py
@@ -518,7 +518,7 @@ class SpinBox(QtGui.QAbstractSpinBox):
         
         # tokenize into numerical value, si prefix, and suffix
         try:
-            val, siprefix, suffix = fn.siParse(strn, self.opts['regex'])
+            val, siprefix, suffix = fn.siParse(strn, self.opts['regex'], suffix=self.opts['suffix'])
         except Exception:
             return False
             
diff --git a/pyqtgraph/widgets/tests/test_spinbox.py b/pyqtgraph/widgets/tests/test_spinbox.py
index 10087881..cff97da7 100644
--- a/pyqtgraph/widgets/tests/test_spinbox.py
+++ b/pyqtgraph/widgets/tests/test_spinbox.py
@@ -18,6 +18,8 @@ def test_spinbox_formatting():
         (12345678955, '12345678955', dict(int=True, decimals=100)),
         (1.45e-9, '1.45e-09 A', dict(int=False, decimals=6, suffix='A', siPrefix=False)),
         (1.45e-9, '1.45 nA', dict(int=False, decimals=6, suffix='A', siPrefix=True)),
+        (1.45, '1.45 PSI', dict(int=False, decimals=6, suffix='PSI', siPrefix=True)),
+        (1.45e-3, '1.45 mPSI', dict(int=False, decimals=6, suffix='PSI', siPrefix=True)),
         (-2500.3427, '$-2500.34', dict(int=False, format='${value:0.02f}')),
     ]
     
@@ -26,3 +28,14 @@ def test_spinbox_formatting():
         sb.setValue(value)
         assert sb.value() == value
         assert pg.asUnicode(sb.text()) == text
+
+        # test setting value
+        if not opts.get('int', False):
+            suf = sb.opts['suffix']
+            sb.lineEdit().setText('0.1' + suf)
+            sb.editingFinishedEvent()
+            assert sb.value() == 0.1
+            if suf != '':
+                sb.lineEdit().setText('0.1 m' + suf)
+                sb.editingFinishedEvent()
+                assert sb.value() == 0.1e-3