The possible contributions of calorimetry to the study of the thermodynamics and kinetics of the process of reversible hydration of the carbonyl group are considered and discussed. Measurements have been carried out of the heat evolved when pyridine-2-, -3-, and -4-carbaldehyde are added to aqueous solutions, neutral and acidic, in the temperature range 5–35°, and values of the enthalpy and entropy changes corresponding to the formation of gem-diols and of conjugated acids of these bases are calculated.