Ag2S nanoparticles were synthesized in situ by thermal decomposition of a single-source precursor (SSP, silver diethyldithiocarbamate) in the pores of porous Vycor glass (PVG). The precursor was incorporated into PVG by sublimation or impregnation of a solution of the precursor. The band gaps of Ag2S in PVGs were estimated to be 1.63 and 1.43 eV for the samples prepared by the decomposition of the precursor incorporated by these two methods, indicating occurrence of quantum size effect in the nanoparticles.