Biodiesel was produced from waste frying oil (WFO) by the two step catalyzed process. The free fatty acids (FFA) were first esterified with methanol catalyzed by SiO2 pretreated with HF. The catalyst was characterized by its acid surface sites, FTIR, SEM, and, its CO2 and H2O adsorption reactivity. This catalyst was easy to recover, was not hydrated or carbonated, was not dissolved by the reactants or the products of the reaction, showed a high activity for the FFA esterification and presented high stability. After 10 esterification runs, the catalyst activity remained unchanged. During the second step, the triglycerides in WFO were transesterified with methanol catalyzed by NaOH. The biodiesel obtained was characterized by its physical properties and its chemical composition. Gas chromatography and mass spectrometry analysis revealed that the process proposed in this investigation led to a biodiesel containing 96% methyl esters. (C) 2010 Elsevier B. V. All rights reserved.