Evaluation of the average heats of ionization ΔHi.) for oxidized and reduced horse heart cytochrome c reveals that the heats of ionization for both redox species as a function of pH are identical within the precision of the method. Since this result is obtained in spite of the differences in the numbers of groups titrated in the two forms of the protein, the redox forms must have the same heats of ionization per group titrated. When the heats of ionization are plotted as a function of the groups titrated, the differences are more clearly seen. For proteins with one or two imidazoyl ionizations between many carboxyl and ε-amino ionizations, there is no clearly indicated plateau and the heats of ionization of the imidazole groups of histidines cannot be obtained by this procedure. The analysis is, however, useful for eliminating heats of ionization which are abnormal since the shape of the calculated curves are at variance with the experimental data.