We have identified calcium oxide as a solid base catalyst for biodiesel production with environmental benignity, but the neutralizing reaction with free fatty acids is a technical hurdle to put the solid base catalyst to practical use. In this paper, removal of free fatty acids by esterification with methanol over acidic solid are studied using H-Y zeolite, sulfated titanium oxide, and cation-exchange resin. Among the employed materials, only cation-exchange resin was appropriate for catalyzing the esterification in used cooking oil. As a result, we found a possibility that the acid-catalyzed esterification could be utilized as the pretreatment of the feedstock for releasing calcium oxide from the catalytic deterioration
