The platinum(IV) hydride complexes Tp(Me2)PtR(2)H (Tp(Me2)= hydridotris(3,5-dimethylpyrazolyl)borate, R = Me (1a), Ph (1b)) react with molecular oxygen to form platinum(IV) hydroperoxide complexes Tp(Me2)PtR(2)OOH (R = Me (2a) and Ph (2b), respectively) in high yield. The results of kinetic and mechanistic studies of these reactions are consistent with the net insertion of molecular oxygen into the Pt(IV)-H bonds occurring via radical chain mechanisms. The radical chain pathways resemble, in many respects, those documented for autoxidations of organic substrates, but significant differences are also evident. The autoxidations of 1a and 1b both autoaccelerate, but the nature of the rate accelerations and the dependence of the rates on the hydroperoxide products are not the same. The different rate laws observed for the reactions of Tp(Me2)PtR(2)H complexes with molecular oxygen can be rationalized on the basis of similar initiation and propagation events with different chain termination steps.