The oxidation of phenetidine with sodium vanadate is proposed for determining phenothiazine derivatives by the kinetic method. The reaction rate was studied depending on the solution acidity and reagent concentrations, and the optimum determination conditions were selected. The analytical range was 3–100 ng/mL for six phenothiazine alkyl derivatives (aminazine, diprazine, promazine, meterazine, tisercine, and triphtazine). The detection limits (3 to 10 ng/mL) depend on the substituent in the phenothiazine molecule. The relative standard deviations were from 3 to 20%.