Highly dispersed gold particles (<2 nm) were synthesized within the pores of mesoporous silica with pore sizes ranging from 2.2 to 6.5 nm and different pore structures (2D-hexagonal, 3D-hexagonal, and cubic). The catalysts were reduced in flowing H-2 at 200 degreesC and then used for CO oxidation at temperatures ranging from 25 to 400 degreesC. The objective of this study was to investigate the role of-pore size and structure in controlling the thermal sintering of An nanoparticles. Our study shows that sintering of An particles is dependent on pore size, pore wall thickness (strength of pores), and pore connectivity. A combination of high-resolution TEM/STEM and SEM was used to measure the particle size distribution and to determine whether the An particles were located within the pores or had migrated to the external silica surface.