Abstract The temperature course of the vapour pressure and the solubility of glassforming systems is constructed, vitrification being considered as a freezing-in phenomenon. Generalized expressions for the vapour pressure of the crystal, the undercooled melt and the glass are obtained. The equilibrium constant and the affinity of heterogenous reactions with the participation of vitreous solids is determined and the kinetics of dissolution and precipitation of glasses are examined. A thorough investigation on the kinetics of dissolution of a simple glassforming model (phenolphthalein) gives for the first time an unambiguous solubility-temperature diagram of a glassforming system. Critical examination of experimental evidence on vapour pressure and the solubility of glasses is shown to be in agreement with therthermodynamic deductions given in the theoretical paragraphs of the paper. The problem of crystal growth at constant temperature with vitreous solids used as a constant source of supersaturation is reexamined and the possibility of the crystallization of a glass via a third phase (solution) below T g is investigated.