Stable monomeric tert-butylperoxo complexes of Pt(II) of the type trans-P2Pt(Rx)(OO-t-Bu) (P = tertiary phosphine; Rx = CF3, Ph-eCN, Ph) have been prepared by condensation reactions of the corresponding hydroxo complexes with t-BuOOH. These have been characterized with conventional spectroscopic techniques: IR and lH, 19F, and , 31P NMR. The X-ray crystal structure of trans-(PPh3)2Pt(Ph)(OO-t-Bu) is also reported, which confirms the end-bonded peroxidic nature of these complexes. The crystals are triclinic, space group Pi with 2 molecular units in a cell and lattice parameters a = 20.11 ( 1 ) A , b = 10.114(7)A, c = 12.024(8)A, α = 103.4(1)°, β = 118.5(1)°, and γ =98.5(1)°. The structure was refined to an R value of 0.045 (Rw = 0.055) for 3335 independent reflections. Spectroscopic investigations with 3lP and 19F NMR suggest a considerable covalent character for the Pt-O bond in both tert-butylperoxo and hydroperoxo species. All trans tert-butylperoxo derivatives except one (P = PBz3) proved effective in the selective oxygenation of l-octene to 2-octanone. The influence on the yield of the oxygenation reaction due to the ancillary phosphines and Rx ligand, along with conductivity and spectroscopic studies, constitutes the basis for a discussion of the possible oxygen-transfer pathways occurring in this system.