The kinetics of the thermal degradation of promethazine in an acidic medium under various conditions were investigated. The degradation of promethazine and the formation of some degradation products were studied under aerobic and anaerobic conditions. The influence of pH, metal ions such as copper(II) and iron (III), and antioxidants was investigated. In an oxygen-saturated medium, promethazine generally followed first-order kinetics. Increasing the pH increased the degradation rate to a limiting value at pH 5. Addition copper (II) increased the degradation rate over the whole process, while iron (III) caused an increase for only a short time. Ascorbic acid sometimes increased the degradation rate, while higher concentrations of hydroquinone also accelerated the degradation. Pyrosulfite did not have any influence. Under anaerobic conditions, promethazine degraded only in the presence of copper (II) and iorn (III) ions. As a result of the studies on the qualitative and quantitative aspects of the oxidation process, a mechanism for the oxidative degradation of promethazine is suggested. Promethazine 5-oxide and a number of degradation products without intact side chains are formed via a semiquinone free radical. The influence of several factors on the degradation process is discussed.