diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index ad7563140b2..59a757f28e3 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -190,6 +190,7 @@ class purchase_order(osv.osv):
         'invoiced': fields.function(_invoiced, string='Invoice Received', type='boolean', help="It indicates that an invoice has been paid"),
         'invoiced_rate': fields.function(_invoiced_rate, string='Invoiced', type='float'),
         'invoice_method': fields.selection([('manual','Based on Purchase Order lines'),('order','Based on generated draft invoice'),('picking','Based on incoming shipments')], 'Invoicing Control', required=True,
+            readonly=True, states={'draft':[('readonly',False)], 'sent':[('readonly',False)]},
             help="Based on Purchase Order lines: place individual lines in 'Invoice Control > Based on P.O. lines' from where you can selectively create an invoice.\n" \
                 "Based on generated invoice: create a draft invoice you can validate later.\n" \
                 "Bases on incoming shipments: let you create an invoice when receptions are validated."