Slow evaporation of aqueous solutions containing mixtures of Na-2[Os(phen)(CN)(4)], Ln(III) salts (Ln = Pr, Nd, Gd, Er, Yb), and (in some cases) an additional ligand such as 1,10-phenanthroline (phen) or 2,2'-bipyrimidine (bpym) afforded crystalline coordination networks in which the [Os(phen)(CN)(4)](2-) anions are coordinated to Ln(III) cations via Os-CN-Ln cyanide bridges. The additional diimine ligands, if present, also coordinate to the Ln(III) centers. Several types of structure have been identified by X-ray crystallographic studies. Photophysical studies showed that the characteristic emission of the [Os(phen)(CN)(4)](2-) chromophore, which occurs at similar to 680 nm in this type of coordination environment with a triplet metal-to-ligand charge transfer ((MLCT)-M-3) energy content of similar to 16 000 cm(-1), is quenched by energy transfer to those Ln(III) centers (Pr, Nd, Er, Yb) that have low-lying f-f states capable of accepting energy from the Os(II)-based (MLCT)-M-3 state. Time-resolved studies on the residual (partially quenched) Os(II)-based luminescence allowed the rates of Os(II) -> Ln(III) energy transfer to be evaluated. The measured rates varied substantially, having values of >5 x 10(8), similar to 1 x 10(8), and 2.5 x 10(7) s(-1) for Ln = Nd, Er or Yb, and Pr, respectively, These differing rates of Os(II) -> Ln(III) energy transfer can be rationalized on the basis of the availability of f-f states of the different Ln(III) centers that are capable of acting as energy acceptors. In general, the rates of Os(II) -> Ln(III) energy transfer are an order of magnitude faster than the rates of Ru(II) -> Ln(III) energy transfer in a previously described series of [Ru(bipy)(CN)(4)](2-)/Ln(III) networks. This is ascribed principally to the lower energy of the Os(II)-based (MLCT)-M-3 state, which provides better spectroscopic overlap with the low-lying f-f states of the Ln(II) ions.