The present investigation offers experimental results concerning the degradation kinetics of the sodium salt of phenylbutazone based on a reliable HPLC procedure. The method allows the simultaneous determination of the parent compound and its main decomposition products. The degradation kinetics at 37 degrees C were compared at pH 7.9 and 10.0 and under oxygen and nitrogen atmospheres. Parallel tests were carried out in the dark and under photolytic conditions for the aforementioned conditions. The influence of traces of iron and a chelating agent of iron on the degradation was studied. At pH 7.9 and pH 10.0 the main degradation products are 3-hydroxy-2-oxohexanoic acid 1,2-diphenylhydrazide and 3-carboxy-2-oxohexanoic acid 1,2-diphenylhydrazide. Azobenzene is formed only at pH 10.0. At pH 7.9, in the dark, the degradation proceeds with a lag phase. In contrast, no lag phase is observed under photolytic conditions. The process of autoxidation and hydrolysis is catalyzed by traces of iron both in the dark and under irradiation conditions. An unexpected increase in the degradation is observed in the presence of iron(III) and EDTA in aerobic conditions and under irradiation.