The stopped-flow technique is used to develop fluorimetric determinative methods for two phenothiazine drugs: perphenazine and chlorpromazine. The methods are based on the oxidation of these compounds to their corresponding fluorescent sulfoxides. Owing to the intrinsic characteristics of this technique, which allow rapid and thorough mixing of reactants, the oxidation is affected by dissolved oxygen itself, which therefore makes an oxidizing reagent unnecessary. Two methods are proposed for each phenothiazine, one based on initial rate and another on maximum fluorescence intensity measurements. Their linear ranges are 0.1–40 μg mL−1and their precision (%RSD) between 0.9 and 2.8%. These methods have been satisfactorily applied to the determination of both phenothiazines in several pharmaceutical preparations. On the basis of the additivity of the initial rate and maximum fluorescence intensity of perphenazine and chlorpromazine, the proportional equations principle is applied to the simultaneous determination of these compounds. Perphenazine/chlorpromazine mixtures in ratios between 1:8 and 8:1 are successfully resolved.