This study concerns the action mechanism of nucleating agents added to sodium acetate trihydrate (melting point 331 K) as a latent heat storage material in order to suppress its inherent problem of supercooling. Such nucleating agents are activated by the solidification of the trihydrate melt, but deactivated by superheating beyond a critical temperature, upper limiting temperature, during the process of heat storage. A previous study (Watanabe, 1992b) on a system of trihydrate melt with sodium oxysalts indicated that the limiting temperatures of these salts were related with their solubilities in water. The present study employed a melt system with orthophosphates in which sodium cations were replaced with H+ (partial substitution), Li+, K+, Ag+ and NH4+, and the limiting temperatures of the monovalent-cation-substituted phosphates were investigated by means of thermal cycle examination. Samples were prepared by adding an excess amount of each of the phosphates to sodium acetate trihydrate, melted by superheating at a temperature of 355 K, and then solidified by forced supercooling to 233 K. Deactivation was observed with all six kinds of phosphates, whose limiting temperatures differed from each other and fell within the range of 12-34 K above the melting point. No remarkable relationship was observed between the limiting temperature and the nucleation temperature of the melt either before or after deactivation. The degree of similarity in the lattice shapes of unit cell planes between the crystals of sodium acetate trihydrate and those of the phosphates was calculated with numerical data defined as root-mean-square deviations of their corresponding axis lengths, but no reasonable interrelation was found between the degree of similarity and the limiting temperature. On the other hand, the solubility in water of trisodium phosphate, the phosphate with the highest limiting temperature, was slightly below lower than that of the trihydrate. Consequently, in agreement with the previous study, it was concluded that an optimal solubility value for elevating the limiting temperature might exist in the melt system with monovalent-cation-substituted phosphates.