The effect of temperature and of humidity on the stability of quinapril hydrochloride (QHCl) in tablets as well as on the durability of the degradation process was investigated. The investigation has been performed by means of the “enhanced ageing” test (in the temperature range from 383 to 333 K, at a relative humidity between and 76%). From the results of the study it follows that QHCl in tablets undergoes decomposition according to a first order reaction model. The activation energies as well as enthalpy and entropy (for temperature 298 K) have also been determined. It is concluded that the decomposition of QHCl in tablets proceeds by way of hydrolysis of the ester group and an intramolecular cyclization.