The porous silica dispersed with silver nanometer particles (about 3 nm) within its pores (cages) has been prepared by a new method. In the process of alternative exposure to the ambient air and annealing, this material assumes reversible transition between transparency and opacity. This can be explained by oxidation of the outer part of the Ag particles during exposure to the ambient air and dissociation of the oxide during annealing in air. ©1996 American Institute of Physics.