Although neither vanadium(V) ions nor hydrogen peroxide efficiently oxidize the title substrates, they do so in combination, with vanadium(V) as the catalyst in acidic aqueous acetonitrile. The kinetic data show that, of the two peroxovanadium species present, OV(O-2)(+) and OV(O-2)(2)(-), only the laffer reacts at a detectable rate. This unanticipated result can be attributed to the weaker 0-0 and V-0 bonds in the diperoxo complex. The rate constants for both series of substrates follow the Hammett correlation, with p(P) = -1.35 and p(S) = -0.83. To analyze properly the kinetic data for the Ar3P compounds, account must be taken of the protonation to Ar3PH+ in acidic solution. In retrospect, our earlier study [Abu-Omar, M. M.; Espenson, J. H. J. Am. Chem. Soc. 1995, 117, 272-280] of phosphine oxidation catalyzed by MeReO3 failed to do so, and the reaction constant must be corrected from the originally reported value to -1.56.