The electrochemical oxidation of three biologically important amines, such as ephedrine, pseudoephedrine and norephedrine, after derivatization with phenylisothiocyanate was investigated by means of cyclic voltammetry on a glassy carbon electrode, The corresponding thioureido derivatives can be oxidized through two one-electron steps at acidic pH values, presumably involving the intermediacy of proton transfers. A mechanistic pathway is also established on the basis of experimental data. Remarkably, this derivatization envisages a novel and advantageous protocol for the separation and subsequent determination of sympathomimetic amines by liquid chromatography coupled with electrochemical detection.