A simple and rapid spectrophotometrie method for the quantitative determination of thioxanthene derivatives; chlorprothixene, thiothixene and methixene either in their pure form or in tablets was developed. The method is based upon, the formation of charge transfer complexes with iodine. The spectra of the complex show maxima at 360 and 275 nm with a high apparent molar absorptivity which facilitates the determination of 1–20 μg.ml−1of the intestigated compounds. The mean recoveries are: 100. 1/2 6 ± 1.38%, 100.32 ± 1.15% and 99.87 ± 1.12% for chlorprothixene, thiothixene and methixene respectively. The method is applied to determine these drugs as single, entities or in certain formulations and the results compare favourably to compendial methods.