The adsorption of Ru(bpy)(3)(2+) (bpy = 2,2’-bipyridine) onto negatively charged antimony-doped SnO2 colloidal particles (ca. 4-nm diameter) was monitored by time-resolved luminescence. Electron injection from photoexcited Ru(bpy)(3)(2+) into the conduction band of the semiconductor particle results in luminescence quenching predominantly within the laser pulse when the complex is bound to the particle, whereas the luminescence of the unbound complex decays with a lifetime of several hundred nanoseconds. The relative concentrations of the bound and free species are thus readily obtained by determining the relative contributions of the quenched and unquenched components in the luminescence decay curve. Binding of Ru(bpy)(3)(2+) to the particles conforms to the Frumkin adsorption isotherm and suggests that the adsorbed Ru(bpy)(3)(2+) molecules self-associate with a free energy of interaction equal to -1.9 +/- 0.7 kcal mol(-1). The average maximal capacity of each particle for adsorbed Ru(bpy)(3)(2+) determined by the binding data, 29 +/- 10, is in agreement with the value obtained by ion exchange, 30 +/- 4, but is smaller than predicted for a close-packed monolayer.