The goal of this study was to determine the kinetic parameters involved in the decomposition of 2-(5-methyl-4-isoxazolylamino)-N-(5-methyl-4-isoxazolyl)-1,4-naphthoquinone-4-imine (1) in aqueous solution and to identify the main degradation products. An isocratic HPLC assay was used to study the degradation rate of 1. The products of hydrolysis were identified by comparison of their retention times with those of authentic samples. The amount of 1 and the two degradation products resulting from storage of 1 in various buffer solutions was followed in function of time by a reversed-phase HPLC stability-indicating method. The observed degradation rates followed pseudo-first-order kinetics at constant pH, temperature and ionic strength. The logk-pH-profile was constructed at 35 degrees C from the first-order rate constants obtained from studies at pH values ranging from 0.88 to 10.80 (mu=0.5 M). Hydrolysis in the acidic and alkaline media resulted in the formation of two degradation products in each case. The pH-rate profile of 1 in buffer solution was adequately described using a four-term rate equation. The obtained pH-rate profile indicated specific acid-base catalysis with a region of maximum stability between pH 6.40 and 7.40 which can be adequate for formulations of 1.