Microporous polysilsesquioxane xerogels were prepared by sol-gel processing of 1,4-bis(triethoxysilyl)benzene and 1,6-bis(triethoxysilyl)hexane. By the BET and BJH methods, the surface areas and average pore diameters of phenyl- and hexyl-bridged polysilsesquioxane (PPS and HPS) xerogels were found to be 531 and 533 m2/g and 34 and 43 angstrom, respectively. These amorphous materials are potential candidates for a novel confinement matrix for the growth of quantum-sized semiconductor particles or metal clusters. The xerogels were treated successively with Cd2+ and S2- ions, resulting in the formation of quantum-sized CdS particles. Confirmation of their identity was established by high-resolution EDAX and by analysis of the resulting electron diffraction pattern. The diameter of quantum-sized CdS doped into PPS was calculated by the Brus method (from the blue shift of the UV absorption band edge of the CdS particles) to be 56 angstrom. HRTEM images permitted direct and independent evaluation of the size distribution of CdS particles. The measured diameters of CdS doped into PPS and HPS were approximately 60 and 90 angstrom, respectively. These findings provide support for the use of amorphous, microporous polysilsesquioxanes as a confinement matrix for the growth of quantum-sized semiconductor particles over a range of particle sizes.