The possibility of obtaining permanent photoinduced refractive index changes, up to -10(-3), in nanostructured silica-based composites has been demonstrated in SiO(2):SnO(2) optical-grade glass ceramics exposed to ultraviolet radiation. Optical and electron paramagnetic resonance data suggest that the negative refractive index change involves a modification at the surface of the SnO2 nanoclusters, leading to a reduction of their crystalline size.