To clarify the inevitability of the relationship between the melting point (T-m= 291 K) and the heat of fusion (Delta H-f=135 kJ/kg) of potassium acetate dihydrate, a potential latent heat storage material, a quantitative examination of the relationship was made in terms of the structural formula. First, four pairs of salts comprising Na+ or K+ with the same given anion were investigated, for all of which the degree of hydration and the dehydration transition temperature of the hydrate was known. It was found that the dehydration temperature of the potassium salt was generally 20.50 K lower than that of the sodium salt. It was also found that the dehydration temperature, that is T-m, was lower as the ratio of Stokes radius of the salt ion in aqueous solution to the ion radius in crystal structure decreased. Next, the entropy change Delta S-f (=Delta H-f/T-m) of the hydrate during fusion was expressed as a combination of entropy change Delta S-LHO of water during fusion and the mass fraction Phi(w) of the water of crystallization in the hydrate. With this combination, a simple relational expression between T-m and Delta H-f was derived. Application of this expression to the measured data for 18 different hydrates, it was indicated that the numerical value of Delta H-f could be estimated with a relative standard deviation of +/- 0.212 when the structural formula and the T-m value of a hydrate were both known. Through this, the inevitability of the correspondence between the Delta H-f value and T-m for potassium acetate dihydrate was also confirmed.