Abstract Properties of tricalcium phosphate and effects of many other ingredients upon the structure and solubility of the phosphate were investigated. By the reaction of calcium salts with phosphates in aqueous solution, tricalcium phosphate hydrate with a definite structure is not formed but hydroxy apatite with an excess of phosphoric acid is formed, which varies to β-tricalcium phosphate by heating between 700–800°C. Contrary to many other reports, it was found that β-tricalcium phosphate was fairly soluble in citric acid. The solubility is reduced remarkably by a small amount of admixtures, especially of magnesia. Magnesia stabilizes the β modification and prevents β→α inversion of the phosphate. The effect of alumina and ferric oxide is similar but not so intense as magnesia. On the contrary, barium oxide stabilizes the α modification. Sodium oxide affects variously according to the ratio of substitution for lime. Constitution and solubility of phosphate fertilizers were discussed in relation to these results.