The objective of this study was to determine the effect of povidone. hydroxypropyl methylcellulose, hydroxypropyl cellulose, lactose and magnesium stearate on the stability of quinapril hydrochloride. The applied HPLC method was validated by the following parameters: selectivity, precision, linearity and accuracy. Binary 1:4 mixtures of quinapril hydrochloride and selected excipients were stable at 318 K/76.4%RH and at 318 K/0%RH. The kinetic parameters of the degradation reaction of quinapril hydrochloride in the presence of excipients were calculated. The degradation of quinapril hydrochloride with acidic excipients was significantly lower than that observed for the basic excipients (magnesium stearate) under the same study conditions. The degradation rate constant was approximately 100 fold greater than the decomposition in the quinapril hydrochloride alone. Thus, the pathway of degradation of quinapril hydrochloride in model drug-excipient mixtures was depended on pH of excipients and humidity.