Synthesis of Pd nanoparticles with controlled size (d(Pd) = 1-3.6 nm) was carried out within the pores of the mesoporous HMS and SBA-15 silicas. Pd was ion-exchanged on non-calcined silicas, prepared by solvent extraction of the templates. A high concentration of silanol groups on the mesopore surface allowed attaining Pd loading up to 4.4. wt.%. The Pd/HMS and Pd/SBA-15 were characterised by chemical analysis, XRD, N2 adsorption-desorption and transmission electron microscopy (TEM) methods. The materials possess a high SSA and narrow pore size distribution. Introduction of Pd nanoparticles in HMS resulted in a progressive loss of the regularity in the mesoporous structure. On the contrary, all Pd/SBA-15 composites retained the original well-ordered 2D hexagonal structure of SBA-15. The thick walls of the SBA-15 framework are accounted for the higher stability observed. The TEM investigations confirmed that the Pd nanocrystals were located within the SBA-15 mesoporous framework channels. The particle size did not exceed the mesopore diameter (2-6 nm) at Pd loading of 0.1-4.4wt.%. Pd clusters were found to be resistant against sintering during air-calcination (550 degreesC, 4h). The catalyst 2.1%Pd/SBA-15 used in methane combustion at 520 degreesC demonstrated stable activity during 6h on stream. (C) 2002 Elsevier Science B.V. All rights reserved.