From a study of the 25℃ isotherm the following factors are evident: Two hydrates of zinc sulphate exist at this temperature, the heptahydrate and the monohydrate. The transition point between the ZnSO₄·7H₂O and ZnSO₄·H₂O is given as 22.25% sulphuric acid and 22.81% zinc sulphate. The transition point between the ZnSO₄·H₂O and anhydrous ZnSO₄ is given as 88.08% sulphuric acid and .22% zinc sulphate. The solubility of the zinc sulphate in the sulphuric acid shows a marked decrease at 46% to 48% sulphuric acid is very low in the 97.25% sulphuric acid having only .03% zinc sulphate. There was no evidence of an acid salt of zinc sulphate being formed at this temperature. / M.S.