A series of inorganic salts was dissolved in poly(propylene oxide) (PPO) oligomers, and the effect of temperature on the salt solubility was analyzed. Some of alkali metal salts were separated out as crystals by heating from PPO oligomers (Mw = 400-4000). The salts, which were crystallized by heating, were revealed to have relatively higher lattice energy. The threshold of the lattice energy of these phase-separated salts decreased with increasing cation radius. The solubility of these salts turned to zero on increasing average molecular weight of PPO. This suggested the considerable contribution of the terminal hydroxyl groups of the PPO to the solubility of these salts. The increase of molecular motion in PPO oligomers with elevating temperature was expected to induce the distortion of the cooperative coordination of PPO to the cation. This might be the reason of the decrease in the solubility of salts in PPO.