The reaction kinetics of ethyl benzoate saponification with sodium hydroxide in a heterogeneous reaction medium was investigated. The rate of reaction in a heterogeneous liquid-liquid system incorporates a chemical kinetics term as well as the mass transfer between both phases. The reaction was carried out in a reaction calorimeter and the overall reaction rate was determined from the obtained thermal profiles. The mass transfer parameters were calculated numerically, taking into account those reaction rate constants determined in our previous study, where the above-mentioned reaction was conducted in a homogeneous reaction medium. The dependence of the mass transfer parameters and reaction rates on different heterogeneity stages and temperatures is described.