The kinetics of the oxidation of three biological thiols (L-cysteine, glutathione, and DL-penicillamine) to their sulfinic and sulfonic acid derivatives by CrOO2+ in aqueous perchloric acid and in the presence of 2-propanol have been studied spectrophotometrically with the aid of the initial-rates method. The kinetic order of the oxidant is 2, whereas that of the reductant is not defined. The acidity of the medium has a slight effect on the initial rates (acid catalysis for both L-cysteine and DL-penicillamine and base catalysis for glutathione). An increase of the ionic strength leads to a rise of the initial rate for both L-cysteine and DL-penicillamine, whereas the initial rate for glutathione is insensitive to the ionic strength. The reactions are inhibited by both 2-propanol and dissolved O-2 and catalyzed by Mn2+, whereas Ce3+ has almost no effect on them. At low 2-propanol concentration and in the absence of Mn2+ the initial rate vs temperature plots have a minimum at around 20 degrees C, whereas in the presence of either concentrated 2-propanol or Mn2+ the Arrhenius law is fulfilled. A single mechanism is proposed for the three reactions involving a CrOO2+/thiol complex, CrOOH2+, and CrO2+ as intermediates. The bimolecular rate constants for the reactions of the intermediate CrO2+ with L-cysteine and DL-penicillamine at 25.0 degrees C have been obtained (around 10(3) M-1 s(-1) in both cases). Some kinetic data for the decomposition of CrOO2+ in the absence of thiol are also given.