Experimental support for the view that trace protic species may play a key role in the catalytic cooxygenation of arylphosphines at [ Pt( PPh,),] under nominally aprotic conditions has been sought but not found. Marked reductions in the catalytic rates are observed when PPh, and PMePh, are oxidised in the presence of trace levels of moisture or alcohols. The inhibiting influence of Ph,PO,H and p-MeC,H,SO,H is even greater. Oxygen uptake for the oxygenation of PMePh,, in benzene at 25 "C, is shown to follow the rate law below and is considered in terms of a generalscheme for the co-oxygenation of arylphosphines at Pto in aprotic solvents. It is suggested that the 'oxygen insertion' step is best interpreted as intramolecular nucleophilic attack on t w o co-ordinated phosphines followed by breakdown of the resulting metallacycle by reductive elimination; a mechanism embracing features which may well occur in a range of co-oxygenations at Group 8 transitionmetal centres.Few of the many examples of catalytic co-oxygenation of arylphosphines (PR,) at metal centres by molecular oxygen have received detailed mechanistic a t t e n t i ~n . , ~ Halpern and co-workers3 reported studies on one such oxygenation in benzene using [Pt(PPh,),] as the catalyst [reaction (l)]. In a 2 PPh, + 0, c R ~~~3 1 * 2 PPh,O later related study, using PMePh, and PMe,Ph as substrates and ethanol-toluene (5 : 1) as solvent, they detected hydroperoxidic intermediates in the protic medium and suggested that these might also be involved in related catalytic processes under nominally aprotic conditions where such species would have to be formed from 'trace protic imp~rities'.~ Such a key mechanistic role for trace impurities could have important implications for the mechanisms of the co-oxygenations of alkenes at rhodium centres"' in which PPh, and terminal alkenes are co-oxygenated to PPh,O and methyl ketones in anhydrous benzene at room temperature [reaction (2), R =