Energy transfer between Ru-II and Os-II polypyridyl complexes covalently attached to polystyrene has been in studied in CH3CN. The polymer is a 1:1 styrene-p-aminomethylstyrene copolymer derivatized by amide coupling with the acid-functionalized metal complexes [M-II(bpy)(2)(bpy-COOH)](PF6)(2) (M-II = Ru-II, Os-II; bpy is 2,2'-bipyridine and bpy-COOH is 4'-methyl-2,2'-bipyridine-4-carboxylic acid). In the resulting polymer [co-PS-CH2NHCO-((Ru11Os5II)-Os-II)](PF6)(32), 11 of, on the average, 16 polymer sites are derivatized by Ru-II and five by Os-II. Photophysical properties compared to the homopolymers [co-PS-CH2NHCO-(Ru-16(II))](PF6)(32) and [co-PS-CH2NHCO-(Os-16(II))](PF6)(32) reveal that excitation at Ru-II is followed by efficient energy transfer to the lower energy Os-II sites with near unit efficiency (95%). Time-correlated single photon counting measurements with picosecond time resolution reveal that quenching of Ru-II* produced adjacent to an Os-II trap site is quenched with an average rate constant = 4.2 x 10(8) s(-1). Ru-II* decay and Os-II* sensitization kinetics are complex because the polymer sample consists of a distribution of individual strands varying in chain length, loading pattern, and number of styryl spacers. The kinetics are further complicated by a contribution from random walk energy migration. An average energy transfer matrix element Of similar to 2 cm(-1) for Ru-II* --> Os-II energy transfer has been estimated by using emission spectral fitting parameters to calculate the energy transfer barrier.