Abstract In this study, sulfated tin oxide was modified with cobalt oxide and aluminium oxide resulting in sulfated cobalt-tin (SO 4 2- /Co 2 O 3 -SnO 2 ) and sulfated aluminium-tin (SO 4 2- /Al 2 O 3 -SnO 2 ) mixed oxide catalysts, respectively. Their catalytic activities for esterification of free fatty acids (FFA) in crude palm oil were investigated. Cobalt-tin and aluminium-tin mixed oxides were prepared by co-precipitation method. The mixed oxides were further impregnated with sulfate ions by immersing in sulfuric acid solution, dried and calcined at 450-500 °C. The properties of SO 4 2- /Co 2 O 3 -SnO 2 and SO 4 2- /Al 2 O 3 -SnO 2 samples were determined by nitrogen adsorption, X-ray diffraction and potentiometric titration. The activities of SO 4 2- /Co 2 O 3 -SnO 2 and SO 4 2- /Al 2 O 3 -SnO 2 catalysts were tested in a stirred-tank reactor equipped with a reflux condenser. Esterification between crude palm oil containing 10-10.2 wt% of FFA and methanol was carried out at various reaction times. The comparison in the catalytic activities of SO 4 2- /Co 2 O 3 -SnO 2 and SO 4 2- /Al 2 O 3 -SnO 2 was made. The results showed that the SO 4 2- /Al 2 O 3 -SnO 2 had higher catalytic activity than SO 4 2- /Co 2 O 3 -SnO 2 in esterification of FFA to produce methyl esters.