Free fatty acids content plays an important role in selecting the appropriate route for biodiesel production. Oils with high content of free fatty acids can be treated by acid esterification where an alcohol reacts with the given oil in the presence of acid catalyst. In the current study, an equivolume blend of crude rubber seed oil and crude palm oil is fed to the reaction with methanol as the alcohol of choice and sulfuric acid. Selected reaction parameters were optimized, using Taguchi method for design of experiments, to yield the lowest free fatty acid content in the final product. The investigated parameters include alcohol to oil ratio, temperature and amount of catalyst. The effect and significance of each parameter were then studied based on the fractional factorial design and verified by additional experiments. The optimum conditions for acid esterification which could reduce the free fatty acid content in the feedstock to lower than 0.6 (95 reduction) were 65 °C, 15:1 methanol to oil ratio (by mole) and 0.5 wt H2SO4 after 3 h of reaction time. Temperature had been found to have the most effect on the reduction of free fatty acids followed by reactants ratio while increasing catalyst amount had nominal effect. © 2010 Elsevier Ltd.