Abstract: A chemical kinetic study of the stability of oxacillin in aqueous solution at 3 dif- ferent temperatures (35℃, 40℃ and 50℃) and with a pH range from 1.44-10.93 was carried out. Concentrations were determind by the iodometric method, the latter being proved by the authors to be well in accord with the microbiological method. The degradation of oxacillin was proved to be a first-order reaction at any given pH. The apparent first-order constant K is K_H^A- H⁺ + K_OH^A-OH^- + Ko + K_HPO4⁼HPO₄⁼ . The activation energy E_a and frequency factors are listed in Table 3. The t_1/2 and t_9/10 (hr) of oxacillin from pH 4.5 to 8.0 at 0℃ and 20℃ are presented in Table 5, where column a is for the solutions without HPO₄⁼ ion catalysis and other general acid-base catalysis, and column b for those solutions made by 1/2 strength of the McIlvaine buffer system.