Crystalline salts of 7 8 Didehydro 4 5 alpha epoxy 17 methylmorphinan 3 6 alpha diol sulfate(2:1) i.e. of morphine sulphate are disclosed and four polymorphic forms of these salts are reported. The invention further relates to a pharmaceutical composition comprising an amount of such a salt and such a salt as a medicament and for the treatment and/or prevention of pain.