The mechanism of oxidation by O-2 of (dpms)-(PtMe)-Me-II(OH2) (1) and (dpms)(PtMe)-Me-II(OH)(-) (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 degrees C was explored using kinetic and isotopic labeling experiments. At pH <= 8, the reaction leads to a C-1-symmetric monomethyl Pt-IV complex (dpms)(PtMe)-Me-IV(OH)(2) (5) with high selectivity >= 97%; the reaction rate is first-order in [(PtMe)-Me-II] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O-2 activation at the Pt-II center to form a Pt-IV hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O-2 than neutral complex 1 (pK(a) = 8.15 +/- 0.02). At pH >= 10, the oxidation is inhibited by OH- ions; the reaction order in [(PtMe)-Me-II] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by hydroperoxide 4. At pH >= 12, formation of a C-1-symmetric dimethyl complex 6, (dpms)(PtMe2)-Me-IV(OH), along with [(dpms)Pt-II(OH)(2)](-) (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a C-s-symmetric intermediate (dpms)(PtMe)-Me-IV(OH)(2) (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; k(H)/k(D) = 0.98 +/- 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A S(N)2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.