0 Melt systems of sodium carbonate decahydrate (melting point T-m,T-0 = 304.5 K, latent heat of fusion Delta Hf,0 = 153 kJ/kg, both measured) and of a salt hydrate eutectic E(P-C) (T-m,T-0 = 298 K, Delta H-f,H-0 = 213 kJ/kg, both also measured) consisting of 0.6 mol/mol sodium carbonate decahydrate and 0.4 mol/mol disodium hydrogen phosphate dodecahydrate are attracting attention as potential latent heat storage materials. Changes in the melting point Tm and the latent heat Delta H-f were measured for the two melt systems with readily soluble additives. By using simplified empirical formulae, the abilities of melting point depression a(me) and latent heat reduction b(me) for each additive were quantified. The numerical values of these abilities were different for each additive, but their ratio b(me)/a(me) was almost the same regardless of the kind of additive, showing an eigenvalue of each melt system. In addition, a comparative study was performed of b(me)/a(me) values including those of the calcium chloride hexahydrate system (Watanabe, 2017) and the disodium hydrogen phosphate dodecahydrate system (Watanabe and Hirasawa, 2018b). It was found that, independent of the kind of melt system, the relative degree of melting point depression [1-(T-m/T-m,T-0)] and the relative degree of latent heat reduction [1-(Delta H-f/Delta H-f,(0))], respectively normalized by T-m,T-0 and Delta H-f,H-0, showed an almost constant numerical relationship represented by the following expression. 1-(Delta H-f / Delta H-f,H-0) = 21.4 [1-(T-m / T-m,T-0)]