Diclofenac sodium is a non-steroidal anti-inflammatory drug widely used in painful and inflammatory diseases. In standard conditions, by exposure to relative humidity even below 60% at 25 degrees C, the anhydrous form DS gives rise to a hydrate species DSH, a tetrahydrate form different from that obtained by crystallisation from water and previously described. The method of preparation and the physico-chemical properties of the hydrate form were investigated. Data from FTIR spectroscopy, X-ray powder diffraction and thermal analysis were used for the identification and the characterisation of DSH. DS and DSH were easily differentiated by their IR spectra, X-ray patterns and thermal behaviour. DSH stability was followed at room temperature over a period of 1 year and under different conditions of temperature to verify the tendency to solid-solid transition and to study its existence range. Solubility and intrinsic dissolution studies were performed to compare the physico-chemical properties of DS and DSH. Differences in solubility and intrinsic dissolution rates were pointed out: these studies showed that DS dissolved faster than DSH. Storage under uncontrolled environmental conditions or contact with water vapour during manufacturing process could thus influence the performance of the final dosage form.