A high-performance liquid chromatography method for the determination of dextromepromazine, levomepromazine sulfoxide and 2-methoxyphenothiazine in levomepromazine samples was developed. The separation of the analytes was achieved within 10min on a stationary phase containing cellulose tris(4-methylbenzoate) as chiral selector. The mobile phase consisted of 0.1% diethylamine in methanol with a flow rate of 1.0mL/min. The method was validated according to the International Council for Harmonization guideline Q2(R1). The detection limits based on a signal-to-noise ratio of 3 were in the range of 0.002 to 0.005μg/mL. The method proved to be precise and accurate in the concentration range of 0.025–1.0 % for levomepromazine sulfoxide and 2-methoxyphenothiazine and 0.025% to 3.0% for dextromepromazine relative to a concentration of 0.1mg/mL of levomepromazine, with the exception of levomepromazine sulfoxide at the 0.1% level. The method was subsequently applied to the analysis of finished pharmaceutical products as well as of reference substances of the European Pharmacopoeia.