Calorimetric heats of proton adsorption on rutile and hematite in solutions of KNO3 at the point of zero charge (p.z.c.) agree well with the corresponding enthalpies derived from the shift of p.z.c. with temperature. The enthalphy of charge formation depends on the nature of the oxide, but is practically independent of surface charge and electrolyte concentration. This observation is consistent with a high capacitance nature of the oxide-solution interface. In the theoretical analysis absence of specific adsorption or chemisorption of ions other than protons has been assumed. The electrostatic contribution to the enthalpy of double layer formation is shown to strongly decrease with increasing inner layer capacitance. The relationships between calorimetry and temperature congruence of surface charge-pH curves are discussed in some detail.