A procedure for the simultaneous kinetic-spectrophotometric determination of levodopa (I) and benserazide (II), from their oxidation reaction with KIO₄ in an acidic medium, is described. Both species instantly oxidize, giving rise to compounds which present maximum values of absorbance close to 400 nm. In the presence of an excess of the oxidizing agent, the levodopa derivative evolves to form the corresponding aminochrome (λ_m=480 nm), while the benserazide derivative decomposes to yield colorless compounds. The appearance of new compounds, with absorption bands in the region of 500–700 nm, is additionally seen upon adding the oxidizing agent to a mixture of I and II. These compounds also evolve decomposing and forming colorless products. In spite of the complexity of the system studied, the calibration by bi-linear partial least squares (PLS) as well as by three-way partial least squares (nPLS) permit the quantification of both analytes with a precision on the order of 0.7% for levodopa and of 1.5% for benserazide. nPLS also allows for the qualitative interpretation of the phenomena which occur. The proposed method is applied to the quantification of I and II in the commercial, pharmaceutical preparation Madopar, using high performance liquid chromatography (HPLC) as the analytical reference technique.