diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index 3b06924cd99..26b8861bae2 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -328,18 +328,32 @@ class purchase_order_line(osv.osv):
 		if partner_id:
 			lang=self.pool.get('res.partner').read(cr, uid, [partner_id])[0]['lang']
 		context={'lang':lang}
-		price = self.pool.get('product.pricelist').price_get(cr,uid,[pricelist], product, qty or 1.0, partner_id, {'uom': uom})[pricelist]
+
 		prod = self.pool.get('product.product').read(cr, uid, [product], ['supplier_taxes_id','name','seller_delay','uom_po_id','description_purchase'])[0]
+		prod_uom_po = prod['uom_po_id'][0]
+		if not uom:
+			uom = prod_uom_po
+		price = self.pool.get('product.pricelist').price_get(cr,uid,[pricelist], product, qty or 1.0, partner_id, {'uom': uom})[pricelist]
 		dt = (DateTime.now() + DateTime.RelativeDateTime(days=prod['seller_delay'] or 0.0)).strftime('%Y-%m-%d')
 		prod_name = self.pool.get('product.product').name_get(cr, uid, [product], context=context)[0][1]
-		res = {'value': {'price_unit': price, 'name':prod_name, 'taxes_id':prod['supplier_taxes_id'], 'date_planned': dt,'notes':prod['description_purchase']}}
+
+		res = {'value': {'price_unit': price, 'name':prod_name, 'taxes_id':prod['supplier_taxes_id'], 'date_planned': dt,'notes':prod['description_purchase'], 'product_uom': uom}}
 		domain = {}
-		if not uom:
-			res['value']['product_uom'] = prod['uom_po_id'][0]
-			if res['value']['product_uom']:
-				res2 = self.pool.get('product.uom').read(cr, uid, [res['value']['product_uom']], ['category_id'])
-				if res2 and res2[0]['category_id']:
-					domain = {'product_uom':[('category_id','=',res2[0]['category_id'][0])]}
+
+		res2 = self.pool.get('product.uom').read(cr, uid, [uom], ['category_id'])
+		res3 = self.pool.get('product.uom').read(cr, uid, [prod_uom_po], ['category_id'])
+		domain = {'product_uom':[('category_id','=',res2[0]['category_id'][0])]}
+		if res2[0]['category_id'] != res3[0]['category_id']:
+			raise osv.except_osv('Wrong Product UOM !', 'You have to select a product UOM in the same category than the purchase UOM of the product')
+
 		res['domain'] = domain
 		return res
+
+	def product_uom_change(self, cr, uid, ids, pricelist, product, qty, uom, partner_id):
+		res = self.product_id_change(cr, uid, ids, pricelist, product, qty, uom, partner_id)
+		if 'product_uom' in res['value']:
+			del res['value']['product_uom']
+		if not uom:
+			res['value']['price_unit'] = 0.0
+		return res
 purchase_order_line()
diff --git a/addons/purchase/purchase_view.xml b/addons/purchase/purchase_view.xml
index 1c9b2a04985..a28c829c749 100644
--- a/addons/purchase/purchase_view.xml
+++ b/addons/purchase/purchase_view.xml
@@ -131,7 +131,7 @@
 						<field name="account_analytic_id"/>
 						<newline/>
 						<field name="product_qty"/>
-						<field name="product_uom"/>
+						<field name="product_uom" on_change="product_uom_change(parent.pricelist_id,product_id,product_qty,product_uom,parent.partner_id)"/>
 						<field name="product_id" colspan="3" on_change="product_id_change(parent.pricelist_id,product_id,product_qty,product_uom,parent.partner_id)" context="partner_id=parent.partner_id,quantity=product_qty,pricelist=parent.pricelist_id,uom=product_uom,warehouse=parent.warehouse_id"/>
 						<field name="name" colspan="3"/>
 						<field name="date_planned"/>