Biodiesel has emerged as a promising renewable and clean energy alternative to petrodiesel. While biodiesel has traditionally been prepared through homogeneous base catalysis, heterogeneous acid catalysis has been investigated recently due to its ability to convert cheaper but high "free fatty acid" content oils such as waste vegetable oil while decreasing production cost. In this work, the esterification of free fatty acid over sulfated zirconia and activated acidic alumina was studied experimentally in a batch reactor. The models of the reaction over the catalysts were developed in two parts. First, a kinetic study was performed using a deterministic model to develop a suitable kinetic expression, and the related parameters were estimated. Second, a stochastic model was developed as a pseudo-verification of the nature of the reaction at the molecular level. The esterification of palmitic acid obeyed the Eley-Rideal mechanism in which palmitic acid and methanol are adsorbed on the surface for SO4/ZrO2-550 degrees C and AcAl2O3, respectively. The coefficients of determination of the deterministic model were 0.98, 0.99, and 0.99 for SO4/ZrO2-550 degrees C at 40, 60, and 80 degrees C, respectively, and 0.99, 0.96, and 0.98 for AcAl2O3 at the same temperature. The deterministic and stochastic models were in good agreement when the number of molecules in the system was large enough. In general for a system consisting of few molecules, the stochastic model is suggested, whereas for a large system, the deterministic continuum model is suggested. If computational effort is not a bottleneck, then the stochastic model could be used in all systems.