Abstract The degradation of the enkephalinase inhibitor thiorphan was studied. Liquid chromatography UV and electrochemical detections and several techniques of spectrometry showed that the degradation was oxidative and generates diastereoisomers of the corresponding disulfide. The degradation kinetics of thiorphan under different conditions of storage have been investigated by high performance liquid chromatography with UV detection. The influence of air, temperature, light and pH was studied. The combination of these factors enabled us to determine the optimal conditions of conservation and to propose a stable injectable form.