Production of biodiesel from waste frying oil by photocatalytic esterification. Use of Cr/SiO2 as highly active catalyst for photocatalytic esterification of FFA. Photocatalytic esterification performed under solar irradiation. Photoactivity due to synergetic electronic interaction between Cr6+ and Cr3+. The process can generate biodiesel of high purity that meets the quality demands.