The choice of waste cooking oil (WCO) as a raw material for biodiesel production is recognized to be an attractive and economic alternative to the use of vegetable oils. However, the presence of free fatty acids, impurities and high viscosity of WCO may require several pretreatments before the transesterification. In this study WCO deacidification by esterification is investigated: the results show how both Amberlyst (R) 46 and Purolite (R) D5081 catalysts maintain their performance in a Carberry reactor (where catalyst is confined to minimize mechanical stress) and a slurry reactor after several recycles. A46 was tested in the free fatty acids esterification of blends of WCO with different ratios of crude rapeseed oil and diesel as a solvent. The results show how both the use of the blends with another oil with lower viscosity and diesel are both beneficial to the reaction rate and to the properties of the finished biodiesel. (C) 2012 Elsevier Ltd. All rights reserved.