A microwave applicator with a 2.45-GHz magnetron generator, which provides 460 W of output power, and waveguides to a stirred-tank reactor with a full volume of 500 mL was designed. The microwave reactor operates at high pressure and high temperature. Esterification of benzoic acid with 2-ethylhexanol was chosen as the model reaction. The esterification was homogeneously catalyzed with sulfuric acid and para-toluene sulfonic acid. As heterogeneous catalysts, Cs2.5H0.5-PW12O40, sulfated ZrO2, Fe-2(SO4)(3), and montmorillonite KSF were used. For the same operating conditions, the rate of esterification was concluded to be the same in both the microwave reactor and conventional experimental setups. The results show that homogeneous catalysts are more effective than heterogeneous catalysts. Fe-2(SO4)(3) was the most effective catalyst among the solid catalysts tested. The conversions in the microwave stirred-tank reactor were in agreement with the conversions predicted on the basis of the kinetic parameters obtained under conventional heating. High-pressure and high-temperature operating conditions allowed the reaction rate of esterification to be greatly increased.