The haloperoxidase-catalyzed in vivo oxidation of thiocyanate ion (SCN-) by H2O2 is important for generation of the antimicrobial hypothiocyanite ion (OSCN-), which is also susceptible to oxidation by strong in vivo oxidizing agents (i.e., H2O2, OCl-, OBr-). We report a detailed mechanistic investigation on the multistep oxidation of excess SCN- with peroxomonosulfate ion (HSO5- in the form of Oxone) in the range from pH 6.5 to 13.5. OSCN- was detected to be the intermediate of this reaction under the above conditions, and a kinetic model is proposed. Furthermore, by kinetic separation of the consecutive reaction steps, the rate constant of the direct oxidation of OSCN- by HSO5- was determined: k(2) = (1.6 +/- 0.1) X 10(2) M-1 s(-1) at pH 13.5 and k(2)(H) = (3.3 +/- 0.1) X 10(3) M-1 s(-1) at pH 6.89. A critical evaluation of the estimated activation parameters of the elementary steps revealed that the oxidations of SCN- as well as the consecutive OSCN- by HSO5- are more likely to proceed via 2e(-)-transfer steps rather than 1e(-) transfer.