Photolysis (380 nm) of trans-Pt(PEt3)(2)(Cl)(OH)(OOH)(4-trifluoromethylphenyl) (1) at -78 degrees C in acetone-d(6) or toluene-d(8) yields HOOOH (16-20%) and trans-Pt(PEt3)(2)(Cl)(4-trifluoromethylphenyl) (2). Also observed in acetone-d(6) are H2O2, (CD3)(2)C(OH)(OOH), and (CD3)(2)C(OOH)(2). Thermal decomposition or room-temperature photolysis of 1 gives O-2, water, and 2. Computational modeling (DFT) suggests two intramolecular hydrogen-bonding-dependent triplet pathways for the photolysis and two possible pathways for the thermolysis, one involving proton transfer from the OOH to the OH ligand and the other homolysis of the PtOOH bond, abstraction of the OH ligand, and decomposition of the resulting H2O3. Trapping studies suggest the latter pathway.