The present study deals with the application of X-ray powder diffraction (XRPD) analysis as a technique for identification of the forms of clopidogrel bisulphate (CLP) present in both the active pharmaceutical ingredients (API) and tablets, specifically addressing the question of whether API converts to another form after 12 months of storage. The investigation of the possibility of phase transitions occurring over a temperature range spanning from room temperature to the melting point, both in tablets and pure CLP, were also performed. Tablet samples were observed for the changes in their structure using polarizing optical microscopy, which was also used to determine the melting points of tablet samples. The results gained during this work confirm that XRPD is applicable for API and tablets testing. This is particularly important if we take into account that the method can be used during stability studies, i.e. in order to test the quality of tablets during the validity period.