The complex [(H(2)Nphen)Ru(bpy)2](2+) (H(2)Nphen = 5-amino-1,10-phenanthroline; bpy = 2,2’-bipyridine was covalently attached to the surfaces of colloidal SnO2 particles (ca. 4-nm diameter) by using cyanuric chloride and (3-aminopropyl)triethoxysilane as coupling agents. Electron injection from the MLCT excited state of the complex, *RuL(3)(2+), into the conduction band of SnO2 and recombination of RuL(3)(3+) with the conduction band electron, e(cb)-(SnO2), were investigated by nanosecond laser flash photolysis (532-nm excitation) and steady : state luminescence techniques. These reactions were also investigated in the analogous system in which Ru(bpy)32+ was adsorbed to unmodified SnO2 particles by electrostatic attraction. Electron injection is rapid compared to recombination and competes effectively with the intrinsic decay of *RuL(3)(2+), So that the quantum efficiency of formation of RUHL (Or Ru(bpy)(3)(3+)) and e(cb)-(SnO2) approaches unity. The lower quantum efficiency observed in the covalently modified sol which was peptized with NH4OH (pH 10.7) compared to the one peptized with CH3COOH (pH 4.0) is attributed to the lower thermodynamic driving force for electron injection at the higher pH. On the other hand, the faster rate of recombination at the higher pH is attributed to electrostatic attraction between RuL(3)(3+) and the particle surface, rather than the difference in driving force. The recombination is a first-order reaction whose time dependence is satisfactorily described by the Kohlrausch relaxation function. The rate of decay of RuL(3)(3+) is insensitive to the presence of dissolved O-2 but is accelerated by phenol, which acts as a reductant.