Biodiesel is a renewable fuel suitable to replace fossil fuel which can help to mitigate the carbon footprint of the transportation sector. Its price is still noncompetitive mainly due to the high price of raw materials (vegetable oils) and the costs of the processes. In order to find an efficient basic heterogeneous catalyst to replace the nowadays industrial caustic catalyst, a cheap CaO material was used to produce biodiesel using semi-refined rape seed oil (RSO). For the same catalyst the catalytic activity was tested with oleic acid acidified oil in order to simulate the use of low price materials with high activity. A linear decay of the Fatty Acid Methyl Ester (FAME) yield was observed, when oil acidity increases due to catalyst neutralization. The XRD and TG data showed that catalyst deactivation can be related with catalyst amorphization what can be related with soap formation. The catalytic tests with acidic waste frying oil (WFO) showed that biodiesel can be produced over CaO catalyst from low value raw materials without significant deterioration of the catalytic performances. (C) 2013 Elsevier BM. All rights reserved.