Excess molar enthalpies were measured using a Mettler-Toledo RC1 calorimetric reactor at 298 K and 0.1 MPa for the quaternary system which may occur during the hydrolysis of acetic anhydride with ethanol (production of acetic acid and ethyl acetate). A novel formula for the excess molar enthalpies of the quaternary system as a function of the molar composition has been developed. It is mainly based on a Margules-type equation which illustrates the quaternary excess enthalpies by taking into consideration the contributions from the corresponding binary and ternary systems. With no solvent present in the reaction system, the thermal behavior may significantly vary with the extent of the reaction taking place. The evolutions of the partial excess enthalpy on the calculation of both the heat of mixing during reaction and the reaction enthalpy were tabulated for various extents of the chemical reaction.