diff --git a/Tests/hybrid36.py b/Tests/hybrid36.py
new file mode 100644
index 0000000..4429e07
--- /dev/null
+++ b/Tests/hybrid36.py
@@ -0,0 +1,57 @@
+import unittest
+
+import propka.hybrid36 as hybrid36
+
+class Hybrid36Test(unittest.TestCase):
+    def testDecode(self):
+        test_values = {
+            "99999": 99999,
+            "A0000": 100000,
+            "0": 0,
+            "9": 9,
+            "A": 10,
+            "  ZZZZY": 43770014,
+            "ZZZZZ": 43770015, # ZZZZZ - A0000 + 100000
+            "a0000": 43770016,
+            "zzzzz": 87440031,
+            "zzzzy": 87440030,
+            "99": 99,
+            "A0": 100,
+            "ZZ": 1035,
+            "zz": 1971,
+            "-99999": -99999,
+            "-A0000": -100000,
+            "-0": 0,
+            "-9": -9,
+            "-A": -10,
+            "-ZZZZY": -43770014,
+            "-ZZZZZ": -43770015, # ZZZZZ - A0000 + 100000
+            "-a0000": -43770016,
+            "-zzzzz": -87440031,
+            "-zzzzy": -87440030,
+            "-99": -99,
+            "-A0": -100,
+            "-ZZ": -1035,
+            "-zz": -1971,
+            "PROPKA": 954495146,
+            "A001Z": 100071,
+            "B0000": 1779616,
+        }
+
+        for k, v in test_values.iteritems():
+            self.assertEqual(hybrid36.decode(k), v)
+
+    def testErrors(self):
+        test_values = [
+            "99X99",
+            "X9-99",
+            "XYZa",
+            "",
+            "-",
+            "!NotOk",
+        ]
+
+        for v in test_values:
+            with self.assertRaises(ValueError) as e:
+                hybrid36.decode(v)
+            self.assertTrue(v in str(e.exception))
diff --git a/propka/atom.py b/propka/atom.py
index b9abec4..31a83dc 100644
--- a/propka/atom.py
+++ b/propka/atom.py
@@ -4,6 +4,7 @@ from __future__ import print_function
 
 import string, propka.lib, propka.group
 
+from . import hybrid36
 
 class Atom:
     """
@@ -68,7 +69,7 @@ class Atom:
 
         if line:
             self.name = line[12:16].strip()
-            self.numb = int( line[ 6:11].strip() )
+            self.numb = int( hybrid36.decode(line[ 6:11]) )
             self.x = float( line[30:38].strip() )
             self.y = float( line[38:46].strip() )
             self.z = float( line[46:54].strip() )
diff --git a/propka/hybrid36.py b/propka/hybrid36.py
new file mode 100644
index 0000000..f5b96af
--- /dev/null
+++ b/propka/hybrid36.py
@@ -0,0 +1,52 @@
+import string
+
+_hybrid36_upper_chars = set(string.ascii_uppercase)
+_hybrid36_lower_chars = set(string.ascii_lowercase)
+_hybrid36_digits = set(string.digits)
+_hybrid36_upper_set = _hybrid36_upper_chars | _hybrid36_digits
+_hybrid36_lower_set = _hybrid36_lower_chars | _hybrid36_digits
+
+def decode(input_string):
+    """
+    Convert an input string of a number in hybrid-36 format to an integer.
+
+    """
+    value_error_message = "invalid literal for hybrid-36 conversion: '%s'"
+
+    original_input_string = input_string
+    input_string = input_string.strip()
+
+    # Manually handle negative sign.
+    if input_string.startswith("-"):
+        sign = -1
+        input_string = input_string[1:]
+    else:
+        sign = 1
+
+    if not len(input_string):
+        raise ValueError(value_error_message % input_string)
+
+    # See http://cci.lbl.gov/hybrid_36/ for documentation on the format.
+
+    num_chars = len(input_string)
+    first_char = input_string[0]
+
+    if first_char in _hybrid36_digits:
+        return sign * int(input_string)
+    elif first_char in _hybrid36_upper_chars:
+        reference = - (10 * 36 ** (num_chars - 1) - 10 ** num_chars)
+        _hybrid36_set = _hybrid36_upper_set
+    elif first_char in _hybrid36_lower_chars:
+        reference = (16 * 36 ** (num_chars - 1) + 10 ** num_chars)
+        _hybrid36_set = _hybrid36_lower_set
+    else:
+        raise ValueError(value_error_message % original_input_string)
+
+    # Check the validity of the input string: ASCII characters should be
+    # either all uppercase or all lowercase.
+    for c in input_string[1:]:
+        if c not in _hybrid36_set:
+            raise ValueError(value_error_message % original_input_string)
+
+    # Convert with the int function.
+    return sign * (int(input_string, 36) + reference)