In recrystallization experiments of sulphadimidine from water. dioxane: and their mixtures aimed at preparing the unstable hydrate. a non-stoichiometric solvate with dioxane C12H14N4O2S.1/5C4H8O2 and a pseudopolymorph with similar X-ray powder diffraction and infrared absorption properties bur different thermal behaviour (DSC. TG) were isolated. In co-crystallization experiments of trimethoprim and sulphadimidine, a monohydrate of the respective 1:2 molecular complex and a methanol solvate of the equimolecular complex were obtained from aqueous ethanol and anhydrous methanol. respectively. The thermal stability of both pseudopolymorphic complexes is reflected by the high DSC and TG onset temperatures of solvent escape from the crystal lattice (T(e)approximate to 163 C and T(e)approximate to 144 C. respectively. for water and methanol) and is consistent with the important role of water in the hydrated crystal packing and the very strong hydrogen-bonding interaction of methanol with a sulphonamido oxygen atom in the methanol solvate crystal.