Small Ag particles or clusters doped mesoporouS SiO2 composite glass were prepared by the sol-gel process combined gamma-irradiation at room temperature and in ambient pressure. High-resolution transmission electron microscope (HRTEM) has revealed the existence of Ag nanoparticles and their growing through annealing treatments. With the particle size changing, an interesting reversible peak shift effect of the surface plasma resonance (SPR) was observed in the optical absorption measurement, which is explained as a comprehensive competitive result of both intrinsic property (size effect) and extrinsic influence from the matrix or the surrounding medium (interface interaction and its induced spill-out effect). (C) 2002 Elsevier Science B.V. All rights reserved.