Variations of molar volume and heat capacity of reactive solids with gas absorption/desorption have been investigated experimentally for CaCl2 . nCH(3)NH(2), CaCl2 . nNH(3) and CaCl2 . nCH(3)OH, which are utilizable for driving gas-solid chemical heat pumps. Molar volume measurements were performed by the gas-displacement method, which we developed to measure the volume of reactive solids, and heat capacity measurements were performed by means of differential scanning calorimetry. Molar volume and heat capacity of a reactive solid increase proportionally with the amount of reactive gas absorbed in it. Analysis of literature data has revealed that this linear relationship holds for various kinds of hydrated and ammoniated salts, and that the increase in volume and heat capacity depends only on the amount of absorbed substance irrespective of the kind of salt.