The biodiesel production from Spanish used frying oils has been studied using two operation flow charts: two-step alkaline transesterification and sequential esterification-transesterification, followed by washing in water in both cases, in order to set out the most suitable operational conditions to achieve the highest FAME percentage in the shortest time. Sequential esterification-transesterification reached slightly better results than two-step alkaline transesterification. The resulting product cannot be called biodiesel as the specifications of EN 14214 Standards have not been met. Specifically, FAME content and kinematic viscosity were outside the requirements because of the chemical modifications which took place in the oil during cooking (presence of polar compounds). The influence of polar compounds on the processes has been studied by means of their analysis in the oil and the product. (C) 2009 Elsevier Ltd. All rights reserved.