The present study describes a new application of ruthenium(II) tris(bipyridine) (Ru(bpy)(3)(2+)) and osmium(II) tris(bipyridine) (Os(bpy)(3)(2+)) as phosphorescent labels for the quantification of surface binding of molecules to gold and silver nanoparticles. The fraction of Rtt(bpy)(3)(2+) and Os(bpy)(3)(2+) that is in solution can be distinguished from the surface-bound fraction by the relative lifetimes and integrated emission yields as determined by time-correlated single-photon counting (TCSPC) spectroscopy. Complementary steady-state measurements were carried out to confirm surface attachment of the phosphorescent label molecules. Although the emission of solutions of Ru(bpy)(3)(2+) and Os(bpy)(3)(2+) is quenched proportional to the concentration of 10 nm Au or 20 nm Ag nanoparticles, the quenching is static and not diffusional quenching observed in Stern-Volmer plots. The results demonstrate that time-resolved spectroscopy provides a rapid method for the measurement of surface binding of labeled molecules on metallic nanoparticles. While steady-state measurements require the preparation of a series of samples with varying quencher concentrations and a reference, the method described herein requires a single sample plus reference. The mechanism for phosphorescence quenching on Au and Ag nanoparticles is discussed in terms of energy and electron transfer theories.