The kinetics of degradation of lisinopril (LIS) in solid phase in presence of humidity at 343, 353, 358 and 363 K has been investigated. Changes of the LIS concentration were followed by an HPLC method with UV detection. The thermodynamic parameters at 293 K of degradation of LIS in solid phase E-a (kJ/mol) = 166 +/- 47; Delta H-not equal (kJ/mol) = 163 +/- 48; Delta S-not equal (J/(K x mol)) = 99 +/- 135 for RH = 76.4% have been calculated. Identification of the product of LIS degradation in solid phase was carried out by HPLC-MS. Diketopiperazine derivative of LIS was the major product of its decomposition in the presence of humidity.