In this study the single-crystal X-ray structure of the solvated species (nifedipine)2. 1,4-dioxane is reported for the first time. Included solvent molecules are located in isolated cavities in the crystal, yielding a very stable solvate. Desolvation of this species involves complete disruption of the crystal structure at the relatively high temperature of 150-153 degrees C, i.e., 50 degrees above the boiling point of 1,4-dioxane, and yields a monoclinic polymorph (Modification I) with a melting point of 174 degrees C. When exposed to an aqueous medium for 48 h, the solvate transformed into a dihydrate. The aqueous solubilities of the above species were in the order: 1,4-dioxane solvate >or= Modification I > dihydrate. The solubility of nifedipine was increased sixfold when transformed into an amorphous form by quenched fusion. This amorphous form was relatively stable at room temperature but converted to Modification I when suspended in water at pH 1. The fused materials also converted to Modification I through an intermediate, Modification III, within 6 days when kept at 40 degrees C for 6 days. XRPD analysis showed that grinding increased the crystallinity of the amorphous form due to partial transformation to Modification I. The pulverized amorphous powder was more stable at 40 degrees C and was approximately three times as soluble as Modification I.