Using solvents has been found not only to overcome the miscibility problem in biodiesel synthesis but also makes it possible to obtain precise kinetic data. However, little is known about the effect of these solvents themselves on the reaction kinetics. This study explored solvent effects on the nature of the catalytic activity of the solid acid tungstated zirconia (WZ) for hydrolysis and transesterification of tricaprylin (TCp), two reactions that can occur during biodiesel synthesis, at 130 degrees C and 12.2 atm in a batch reactor using different solvents. It was found that hexane and a mixture of lauric acid (HLa) and hexane resulted in the highest catalytic activities for transesterification and hydrolysis, respectively, probably due to their non-polar and low kinematic viscosity inducing characteristics. Tetrahydrofuran (THF), in contrast, having a higher polarity, tended to reduce the activity for both hydrolysis and transesterification by competing for active sites on the WZ surface and being involved in side reactions with methanol. (C) 2010 Elsevier By. All rights reserved.