Two photometric methods have been described for the assay of three phenothiazine drugs, isothipendyl hydrochloride, dimethothiazine mesylate & trimeprazine tartarate in their tablets. The first is the differential tartarate in their tablets. The first is the differential spectro-photometric method & based on the measurement of the absorbance (Δ A-), first derivative (Δ D1−) or second derivative (Δ D2−) values of the sulphoxide product of phenothiazine versus the intact. The second is based on the absorbance measurement of the pink coloured complex formed in a ratio of two drug moles t o one palladium ion. All these values are following Beer's law, which pernits the phenothiazines determination in their tablets with high accuracy. Using t-test & F-test the results of different methods are of equal accuracy and reproducibility. Both methods complement each ot her during the routine analysis and quality control of the investigated phenothiazine drugs.