The photolysis of the allylplatinum(IV) complex [PtBr(C3H5)(4-MeC6H4)(2)(bipy)], 1, bipy = 2,2'-bipyridine, in air yielded [{PtBr(4-MeC6H4)(2)(bipy)}(2)(mu-O-2)], 2, the first diplatinum-(IV) complex containing a single bridging peroxide ligand. The PtO-OPt bond distance in 2 is 1.481(3) A. Complex 2 is thought to be formed by homolysis of the allyl-platinum bond of 1, followed by reaction of the platinum(III) intermediate[PtBr(4-MeC6H4)(2)(bipy)] with oxygen.