Abstract: The invention concerns certain crystalline solvates and hydrates of the compound of the Formula I which is known inter alia by way of the code number ZD1839. In particular, the invention concerns a first solvate that occurs in the presence of methanol which is designated as Form 2 ZD1839 MeOH solvate, a second solvate that occurs in the presence of dimethyl sulphoxide which is designated as Form 3 ZD1839 DMSO solvate and a trihydrate that occurs in the presence of water which is designated Form 5 ZD1839 trihydrate. The invention further concerns processes for the preparation of these solvates and the trihydrate and for their conversion back to the compound ZD1839, pharmaceutical compositions containing them and their use in the manufacture of medicaments for use the production of an anti-proliferative effect in a warm-blooded animal such as man.