In the decomposition of raw phosphates with sulphuric acid, or with another acid with simultaneous or subsequent addition of the sulphuric acid radicle, the reaction mass is subjected to such a solution vapour pressure that the calcium sulphate present as semihydrate and/or anhydrite becomes sufficiently stable to permit washing or treatment with water, diluted phosphoric acid &;c. without substantial change of the contents of water of hydration. The heating may be carried out in the presence of added phosphoric acid or previously produced reaction mass containing it, and the required temperature may be ascertained from the relation 2p + t = at least 186, where p is the percentage of P2O5 by weight present, and t the temperature necessary. By having a sufficient concentration of phosphoric acid, the decomposition and stabilization may be carried out in an open vessel; for instance the phosphoric acid added may contain at least 36 per cent by weight of P2O5 and at least as much be added as is formed in the decomposition. The addition may be made to the raw phosphate and reaction be effected therewith prior to addition of sulphuric acid. In effecting decomposition with sulphuric acid, excess acid may first be used at the temperature of stabilization, and then further raw phosphate be added; or the excess sulphuric acid may be added to reaction mixture already produced, and the temperature be then raised to that necessary prior to addition of the further raw phosphate required. By applying the process to the direct formation of superphosphate, hardening during storage is prevented or retarded. Specification 314,976, [Class 1 (ii), Inorganic compounds, other than &;c.], is referred to.