Abstract A kinetic-photometric method for the individual determination of imipramine and desipramine and a differential kinetic method for the resolution of mixtures of these tricyclic antidepresant based on the stopped-flow mixing technique were developed for the first time and applied to the determination of these species in serum. Both methods rely on the oxidative coupling of these compounds with 3-methylbenzothiazolin-2-one hydrazone in the presence of ammonium iron(III) sulphate in hydrochloric acid. On the basis of the initial rate and absorbance amplitude of the kinetic curves obtained for each analyte, the resolution of the mixture was accomplished by using the proportional equation method. The linear range of the calibration graphs was 0.3-7.0 μ ml −1 for impramine and 0.3-5.0 μg ml −1 for desipramine. Mixtures of imipramine and desipramine in ratios between 2:1 and 1:8 were satisfactory resolved. The determination of these analytes in serum called for a prior separation step for matrix removal.