diff --git a/doc/source/changelog.rst b/doc/source/changelog.rst
index 75fe024f..010c3879 100644
--- a/doc/source/changelog.rst
+++ b/doc/source/changelog.rst
@@ -5,6 +5,14 @@ MontePy Changelog
 0.5 releases
 ============
 
+#Next Release#
+--------------
+
+**Bug Fixes**
+
+* Fixed parsing error with not being able to parse a blank ``sdef`` (:issue:`636`).
+* Fixed parsing error with parsing ``SSW`` (:issue:`639`).
+
 0.5.3
 --------------
 
diff --git a/montepy/input_parser/data_parser.py b/montepy/input_parser/data_parser.py
index 57635454..63cc0d8b 100644
--- a/montepy/input_parser/data_parser.py
+++ b/montepy/input_parser/data_parser.py
@@ -38,9 +38,9 @@ def data_input(self, p):
 
     @_(
         "classifier_phrase",
-        "classifier_phrase KEYWORD padding",
+        "classifier_phrase MODIFIER padding",
         "padding classifier_phrase",
-        "padding classifier_phrase KEYWORD padding",
+        "padding classifier_phrase MODIFIER padding",
     )
     def introduction(self, p):
         ret = {}
@@ -49,8 +49,8 @@ def introduction(self, p):
         else:
             ret["start_pad"] = syntax_node.PaddingNode()
         ret["classifier"] = p.classifier_phrase
-        if hasattr(p, "KEYWORD"):
-            ret["keyword"] = syntax_node.ValueNode(p.KEYWORD, str, padding=p[-1])
+        if hasattr(p, "MODIFIER"):
+            ret["keyword"] = syntax_node.ValueNode(p.MODIFIER, str, padding=p[-1])
         else:
             ret["keyword"] = syntax_node.ValueNode(None, str, padding=None)
         return syntax_node.SyntaxNode("data intro", ret)
@@ -183,6 +183,7 @@ class ParamOnlyDataParser(DataParser):
     debugfile = None
 
     @_(
+        "param_introduction",
         "param_introduction spec_parameters",
     )
     def param_data_input(self, p):
diff --git a/montepy/input_parser/tokens.py b/montepy/input_parser/tokens.py
index a62856c2..8c5a2220 100644
--- a/montepy/input_parser/tokens.py
+++ b/montepy/input_parser/tokens.py
@@ -26,6 +26,7 @@ class MCNP_Lexer(Lexer):
         LIBRARY_SUFFIX,
         LOG_INTERPOLATE,
         MESSAGE,
+        MODIFIER,
         MULTIPLY,
         NUM_INTERPOLATE,
         NUM_JUMP,
@@ -75,50 +76,6 @@ class MCNP_Lexer(Lexer):
         "cosy",
         "bflcl",
         "unc",
-        # materials
-        "gas",
-        "estep",
-        "hstep",
-        "nlib",
-        "plib",
-        "pnlib",
-        "elib",
-        "hlib",
-        "alib",
-        "slib",
-        "tlib",
-        "dlib",
-        "cond",
-        "refi",
-        "refc",
-        "refs",
-        # volume
-        "no",
-        # sdef
-        "cel",
-        "sur",
-        "erg",
-        "tme",
-        "dir",
-        "vec",
-        "nrm",
-        "pos",
-        "rad",
-        "ext",
-        "axs",
-        "x",
-        "y",
-        "z",
-        "ccc",
-        "ara",
-        "wgt",
-        "tr",
-        "eff",
-        "par",
-        "dat",
-        "loc",
-        "bem",
-        "bap",
     }
     """
     Defines allowed keywords in MCNP.
@@ -471,6 +428,7 @@ class DataLexer(ParticleLexer):
         JUMP,
         KEYWORD,
         LOG_INTERPOLATE,
+        MODIFIER,
         MESSAGE,
         MULTIPLY,
         NUMBER,
@@ -486,6 +444,73 @@ class DataLexer(ParticleLexer):
         ZAID,
     }
 
+    _KEYWORDS = set(MCNP_Lexer._KEYWORDS) | {
+        # ssw
+        "sym",
+        "pty",
+        "cel",
+        # ssr
+        "old",
+        "new",
+        "col",
+        "wgt",
+        "tr",
+        "psc",
+        "poa",
+        "bcw",
+        # materials
+        "gas",
+        "estep",
+        "hstep",
+        "nlib",
+        "plib",
+        "pnlib",
+        "elib",
+        "hlib",
+        "alib",
+        "slib",
+        "tlib",
+        "dlib",
+        "cond",
+        "refi",
+        "refc",
+        "refs",
+        # volume
+        "no",
+        # sdef
+        "cel",
+        "sur",
+        "erg",
+        "tme",
+        "dir",
+        "vec",
+        "nrm",
+        "pos",
+        "rad",
+        "ext",
+        "axs",
+        "x",
+        "y",
+        "z",
+        "ccc",
+        "ara",
+        "wgt",
+        "tr",
+        "eff",
+        "par",
+        "dat",
+        "loc",
+        "bem",
+        "bap",
+    }
+
+    _MODIFIERS = {
+        "no",  # VOLUME
+    }
+    """
+    A keyword flag at the beginning of a input that modifies it's behavior.
+    """
+
     @_(r"([|+\-!<>/%^_~@\*\?\#]|\#\d*)+")
     def PARTICLE_SPECIAL(self, t):
         """
@@ -493,6 +518,17 @@ def PARTICLE_SPECIAL(self, t):
         """
         return t
 
+    @_(r"[+\-]?[0-9]*\.?[0-9]*E?[+\-]?[0-9]*[ijrml]+[a-z\./]*", r"[a-z]+[a-z\./]*")
+    def TEXT(self, t):
+        t = super().TEXT(t)
+        if t.value.lower() in self._KEYWORDS:
+            t.type = "KEYWORD"
+        if t.value.lower() in self._MODIFIERS:
+            t.type = "MODIFIER"
+        elif t.value.lower() in self._PARTICLES:
+            t.type = "PARTICLE"
+        return t
+
 
 class SurfaceLexer(MCNP_Lexer):
     """
diff --git a/tests/inputs/test_complement_edge.imcnp b/tests/inputs/test_complement_edge.imcnp
index e83b83fc..975379a1 100644
--- a/tests/inputs/test_complement_edge.imcnp
+++ b/tests/inputs/test_complement_edge.imcnp
@@ -33,5 +33,5 @@ m814    26000.55c 5.657-2
         28000.50c 9.881-3
         24000.50c 1.581-2
         25055.51c 1.760-3
-
+sdef cel=2 pos=0 0 0 rad=d3 ext=d3 axs=0 0 1
 
diff --git a/tests/inputs/test_dos.imcnp b/tests/inputs/test_dos.imcnp
index c2725156..b6b6b14a 100644
--- a/tests/inputs/test_dos.imcnp
+++ b/tests/inputs/test_dos.imcnp
@@ -46,5 +46,7 @@ ksrc 0 0 0
 kcode 100000 1.000 50 1050
 phys:p j 1 2j 1
 mode n p
+ssw cel=5 99
+sdef $ blank one from issue #636
 
 
diff --git a/tests/test_integration.py b/tests/test_integration.py
index c569c31c..44d4a430 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -51,7 +51,7 @@ def test_original_input(simple_problem):
 
 def test_original_input_dos():
     dos_problem = montepy.read_input(os.path.join("tests", "inputs", "test_dos.imcnp"))
-    cell_order = [Message, Title] + [Input] * 16
+    cell_order = [Message, Title] + [Input] * 18
     for i, input_ob in enumerate(dos_problem.original_inputs):
         assert isinstance(input_ob, cell_order[i])