The localization of metal and alloy particles in monometallic Pd/SiO2 and bimetallic Pd-Ag/SiO2 sol-gel catalysts prepared by cogelation was examined by rotating transmission electron microscopy. The analysis of the resulting micrograph series provides a direct proof that the 2-to 3-nm active metal nanoparticles in those catalysts are located inside microporous silica particles or aggregates. Such localization has long been suspected but with, up to now, no direct evidence. (c) 2006 Elsevier Inc. All rights reserved.