A statistical mechanical treatment is presented for determining the heat capacity and enthalpy changes with temperature in conformational biomolecular (e.g. folding-unfolding of proteins) transitions. The theory is formulated in terms of subunit partition functions, which are the average partition functions of the residues modulated by their interactions with each other and the solvent. The theory is specialized to a system of two types of conformants: type A (for example, protein in the folded state) consisting of subunits of type "a", and type B (for example, protein in the unfolded state) consisting of subunits of type "b", assuming complete cooperativity. It is shown that for such a model, the heat capacity and enthalpy functions obtained from the isothermal-isobaric partition function of the biomolecular system can be calculated from the ratio of the partition functions of the "a" and "b" subunits, their temperature derivatives, and the difference between the lowest enthalpy levels of the subunits. The temperature variation of the partition functions of the subunits can be evaluated from thermal data of the pure conformants A and B outside the transition range. The other parameters may be inferred from the population ratio of the conformants inside the transition region. The theory is applied to lysozyme of pH 2.0, pH 2.25, and pH 3.5, using published optical density data of the conformants within the transition range, and heat capacity data of the pure conformants outside the transition range. Kidokoro and Wada published heat capacity data for lysozyme at pH 2.0 with which we compared our calculated new capacity. The calculated heat capacity curve agrees well with the published one.