diff --git a/array2xls/gui.py b/array2xls/gui.py
index d7d444d..9efb7be 100644
--- a/array2xls/gui.py
+++ b/array2xls/gui.py
@@ -2,7 +2,7 @@ import os
 import tkinter as tk
 import tkinter.ttk as ttk
 from tkinter import filedialog
-
+import pandas
 
 import validators
 import inout
@@ -180,7 +180,7 @@ class Application(tk.Frame):
         if state not in (APP_STATE_3, APP_STATE_4):
             return self.reset()
         if state == APP_STATE_3:
-            text = self._convert_to_single_files()
+            text = self._convert_to_separate_files()
         else:
             text = self._combine_data_files()
         self.status_panel.set_text(text + ' Done')
@@ -197,12 +197,12 @@ class Application(tk.Frame):
         else:
             return APP_STATE_2
 
-    def _convert_to_single_files(self):
+    def _convert_to_separate_files(self):
         total = len(self.validated_files)
-        text = 'Converting data file %d of %d to single excel file ...'
+        text = 'Converting data file %d of %d to separate excel file ...'
+        all_fields = self.file_validator.id_fields + self.file_validator.data_fields
         for i, data_file in enumerate(self.validated_files):
             self.status_panel.set_text(text % (i+1, total))
-            all_fields = data_file.validator.id_fields + data_file.validator.data_fields
             data_frame = inout.read_data_file(data_file, all_fields)
             data_frame = data_frame.set_index(data_file.validator.id_fields)
             name, ext = os.path.splitext(data_file.path)
@@ -211,8 +211,33 @@ class Application(tk.Frame):
         return text % (i+1, total)
 
     def _combine_data_files(self):
-        pass
-
-
-
-
+        total = len(self.validated_files)
+        text = 'Combining data file %d of %d to single excel file ...'
+        data_fields = [self.file_validator.data_fields[i] for i in self.field_panel.listbox.curselection()]
+        selected_fields = self.file_validator.id_fields + data_fields
+        data_frame_list = []
+        for i, data_file in enumerate(self.validated_files):
+            self.status_panel.set_text(text % (i + 1, total))
+            data_frame = inout.read_data_file(data_file, selected_fields)
+            data_frame['File Name'] = os.path.basename(data_file.path)
+            data_frame_list.append(data_frame)
+        master_frame = pandas.concat(data_frame_list, ignore_index=True)
+        pivoted_df = master_frame.pivot(self.file_validator.id_fields[0],'File Name')
+        col_grouper = dict()
+        value_columns = []
+        for value_col, file_col in pivoted_df.columns:
+            if value_col not in col_grouper:
+                col_grouper[value_col] = []
+                value_columns.append(value_col)
+            col_grouper[value_col].append(file_col)
+        sorted_col_names = []
+        for value_col in value_columns:
+            sorted_file_cols = sorted(col_grouper[value_col], key=lambda x: inout.natural_sort(x))
+            sorted_col_names.extend([(value_col, file_col) for file_col in sorted_file_cols])
+        sorted_df = pivoted_df.reindex_axis(sorted_col_names, axis=1)
+        first_file = self.validated_files[0]
+        directory = os.path.dirname(first_file.path)
+        pseudo_extension = self.file_validator.extension.replace('.', '_')
+        xls_path = os.path.join(directory, 'combined' + pseudo_extension + '.xlsx')
+        inout.write_excel_file(xls_path, sorted_df)
+        return text % (i + 1, total)
diff --git a/array2xls/inout.py b/array2xls/inout.py
index ef7f00b..1e914ea 100644
--- a/array2xls/inout.py
+++ b/array2xls/inout.py
@@ -1,8 +1,15 @@
 import os
+import re
 import pandas
 
 import validators
 
+
+RE_NATURAL_SORT = re.compile('([0-9]+)')
+convert = lambda text: int(text) if text.isdigit() else text.lower()
+natural_sort = lambda item: [convert(c) for c in RE_NATURAL_SORT.split(item)]
+
+
 def read_data_file(data_file, usecols):
     with open(data_file.path, 'r', encoding='UTF-8') as file_handle:
         return pandas.read_csv(
@@ -11,4 +18,4 @@ def read_data_file(data_file, usecols):
 
 
 def write_excel_file(path, data_frame):
-    data_frame.to_excel(path)
\ No newline at end of file
+    data_frame.to_excel(path)
diff --git a/array2xls/validators.py b/array2xls/validators.py
index 6bd96a9..ecf2268 100644
--- a/array2xls/validators.py
+++ b/array2xls/validators.py
@@ -70,7 +70,7 @@ dat_validator = Validator(
     ['Dot_Number', 'Probe_Name', 'Gene_Name', 'Col', 'Row'],
     ['X[Pix]', 'Y[Pix]', 'DX[Pix]', 'DY[Pix]', 'Spot_Diameter', 'ROI_Width', 'ROI_Heigth', 'Pixels', 'Bkg',
      'Bkg_SD', 'Net_Signal', 'Net_Signal_SD', 'Net_Integral', 'Net_Integral_SD', 'Acc_Number', 'Proc_Control'],
-    ['Bkg', 'Bkg_SD', 'Net_Signal', 'Net_Signal_SD', 'Net_Integral', 'Net_Integral_SD'] )
+    ['Bkg', 'Net_Signal', 'Net_Integral'] )
 
 csv_validator = Validator(
     'Sensovation Data Files', '.csv', validate_csv,