A reliable equation for the excess heat capacity C-V(E) at constant volume is derived from Guggenheim’s quasi-chemical lattice theory of liquid mixtures for the case of equal-sized molecules, with the assumption that the interchange energy depends on the temperature. Using suitable parameters, a W-shaped concentration dependence for C-V(E) is predicted. The availability of this exact quasi-chemical equation permits the study of the theoretical dependence of C-V(E) On temperature and concentration, a comparison with previous approximate results and better analysis of the molecular meaning of the W-shapes found in the experimental excess heat capacities C-p(E) at constant pressure. Moreover, within this theory, the long-wavelength limit of the Bhatia- Thornton concentration-concentration partial structure factor S-cc(0) is also derived and its correlation with C-V(E) or C-p(E) is discussed.