Pretreatment of the high free fatty acid (FFA) rubber seed oil from Malaysia (RSOM) and Vietnam (RSOV) via esterification reaction has been investigated. Response surface methodology analysis using central composite design was used to optimise important parameters, including reaction temperature, catalyst loading, methanol-to-oil molar ratio and reaction time on FFA reduction. Optimal esterification conversion was achieved at 50 �C, 1.38 wt, 15.98:1 molar ratio and 2 h for RSOM with 99.3 FFA reduction, whereas 65 �C, 10.74 wt, 10:1 molar ratio and 1 h for RSOV with 98.6 FFA reduction. Catalyst loading had been found to have the most effect on the FFA reduction followed by methanol-to-oil molar ratio while increasing temperature and reaction time had nominal effect. The frequency factor and activation energy of RSOV were about 1.8- and 1.2-fold higher than RSOM. © 2015, King Fahd University of Petroleum & Minerals.