The slow oxidation reactions of cysteine, tyrosine and tryptophan by hexacyanomanganate(IV) were used to investigate partial least squares calibration (PLS) as a method to be applied to the kinetic determination of individual species. All the three reactions are first order in hexacyanomanganate(IV) concentration and the reactions with cysteine and tyrosine are first order in amino acid concentration. The stoichiometry was studied in each case. The determination results were always compared with those obtained by applying the initial rate method to the same kinetic runs. When suitable kinetic curves are obtained and the initial rate is easily determined, both methods give similar results; however, when the initial rate is not easily determined (too fast reaction, slope that changes very quickly, etc.) the results obtained with PLS are better (better precision and wider dynamic range). Moreover, and in contrast to the initial rate method, PLS allowed tryptophan to be determined in the presence of some different feed matrices, provided the calibration set was of the same nature as the samples. This advantage is of general application and makes the analytical method more robust. PLS was tested on uncatalysed reactions but it could equally be applied to catalysed reactions.