A study was made of the kinetics of sunflower oil methanolysis. This reaction yields methyl esters (biodiesel) and glycerol and consists of three consecutive reversible reactions. Diglycerides and monoglycerides are intermediate products. A reaction mechanism was proposed involving an initial region of mass transfer control followed by a second region of kinetical control. An analysis was also made of the effects of impeller speed, temperature, and catalyst concentration on the reaction rates, determining the reaction rate constants and the activation energies. The reactions were performed in a batch stirrer reactor, and the reaction mixture was analyzed by gas chromatography. The initial mass transfer-controlled region was not significant using 600 rpm. The kinetically controlled section followed a second-order mechanism for the forward and reverse reactions, where the reaction system could be described as a pseudo-homogeneous catalyzed reaction. The temperature and catalyst concentration increased the reaction rates. The proposed mathematical model fitted the experimental results well.