diff --git a/src/utils/read_source.py b/src/utils/read_source.py
index a7cb801c7f..ab7ce73ea0 100755
--- a/src/utils/read_source.py
+++ b/src/utils/read_source.py
@@ -15,16 +15,18 @@ class SourceFile(object):
 
         for i in range(self.n_sites):
             # Read position, angle, and energy
-            uid = struct.unpack('q', self.f.read(8))
+            (uid,) = struct.unpack('q', self.f.read(8))
+            (weight,) = struct.unpack('d', self.f.read(8))
             xyz = struct.unpack('3d', self.f.read(24))
             uvw = struct.unpack('3d', self.f.read(24))
-            E = struct.unpack('d', self.f.read(8))
+            (E,) = struct.unpack('d', self.f.read(8))
 
             # Create source site and append to list
-            self.sites.append(SourceSite(xyz, uvw, E))
+            self.sites.append(SourceSite(weight, xyz, uvw, E))
 
 class SourceSite(object):
-    def __init__(self, xyz, uvw, E):
+    def __init__(self, weight, xyz, uvw, E):
+        self.weight = weight
         self.xyz = xyz
         self.uvw = uvw
         self.E = E