A method for immobilizing dendrimer encapsulated nanoparticles (DENs) in a sol–gel silica matrix was developed. To evaluate timing effects for DEN addition to the sol–gel preparation, platinum DENs were added at two different times: (1) just before the condensation reactions were initialized, and (2) 24 h after initializing the condensation reactions after the sol formation but before solutions had gelled. After gellation, drying, and activation, the catalysts had similar activities for CO oxidation catalysis. The catalyst prepared after sols had formed, however, was substantially less active for toluene hydrogenation. Infrared spectroscopy of adsorbed CO indicated that this latter catalyst had fewer available surface sites and that both catalysts remained poisoned by dendrimer decomposition products after activation.