The production of biodiesel from waste frying oil (WFO) by a two-step process was investigated. In the first step, the free fatty acids (FFAs) present in the oil were esterified with methanol by a photocatalytic process using Cr/SiO2 semiconductor composite and solar irradiation as light source. The catalytic performances of Cr/SiO2 calcined at different temperatures (500-1000 degrees C) were investigated in FFAs photo-esterification process. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy (DRS), and nitrogen physisorption. Characterization results revealed the presence of Cr6+ in the samples calcined at temperatures between 500 and 600 degrees C. However, a mixture of Cr6+ and Cr3+ was detected in the samples calcined at higher temperatures (700-1000 degrees C). The catalysts calcined between 700 and 1000 degrees C showed a very high activity for FFAs photo-esterification. A mechanism is proposed to explain this high photoactivity, assuming a synergetic electronic interaction between Cr6+ and Cr3+ during photo-irradiation, which generates H+, CH3O center dot and R-COON center dot radicals in high concentration at the photocatalyst surface. The second step consisted in the triglycerides transesterification with methanol through thermal activation using solar irradiation as heating source and catalyzed by NaOH. Produced biodiesel by this process, fulfills all the international requirements for its utilization as a fuel. (C) 2016 Elsevier B.V. All rights reserved.