The reaction of PdMe 2 (bpy) (bpy = 2,2 ' -bipyridine) with diaroyl peroxides is a complex process in which initial oxidation of Pd(II) is followed by methyl group exchange between an undetected Pd(IV) intermediate and PdMe 2 (bpy) to give PdMe(O 2 CAr)(bpy) and PdMe 3 (O 2 CAr)(bpy); the latter Pd(IV) complex decomposes by elimination of Me-Me to give additional PdMe(O 2 CAr)(bpy) which reacts with (ArCO 2 ) 2 to give Me-O 2 CAr and Pd(O 2 CAr) 2 (bpy). The complexes Pd(O 2 CPh) 2 (L 2 ) (L 2 =bpy, N,N,N ' ,N ' -tetramethylethylyenediamine) have been characterised by X-ray diffraction.