The production of methyl esters (biodiesel) from free fatty acids (FFAs) contained in vegetable oils was studied using a heterogeneous acid catalyst. The feedstock was a by-product of a vegetable oil refinery. The experiments were performed in a batch reactor, in a temperature range of 363.15–393.15 K, with an initial molar ratio of methanol to FFAs of 6.6/1, while the catalyst mass was fixed at 2 wt % of the total vegetable oil mass. A technical kinetic model has been developed which accounts for the reversible esterification reaction. Kinetic parameters were determined by fitting experimental data to the model.