Abstract: The attribution of two crystalline modifications of thiamine hydrochloride, their conditions of formation, stability and the mechanism of transformation were investigated physicochemically by phase diagram. It was found that the two crystalline modifications were monohydrate and semihydrate respectively. The water solubility of the former was greater than that of the latter. During recrystallization the direction of crystallization favors the monohydrate adduct form owing to the great difference of crystal growth rate. It dissolved gradually on ageing and separated out as semihydrate adduct form. In moist atmosphere, a"liquid film" was formed around the surface of the monohydrate form due to deliquescence or adsorption of multimolecular layer. Transformation of the polymorphic forms similar to that in ageing process occured in the"liquid film" and was accompanied by caking of the crystals.