Monolithic organic aerogels were prepared by the so]-gel procedure from the polymerisation reaction of resorcinol and formaldehyde in water. The organic aerogels were heat treated in inert atmosphere at either 500 or 1000degreesC to obtain the carbon aerogels. The catalysts were prepared by impregnation with an aqueous solution of [Pt(NH3)(4)]Cl-2 or by dissolving this salt in the initial aerogel mixture. Supported catalysts were pretreated in He at 400degreesC or H, at 300degreesC before their characterization by H-2 chemisorption, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy or before testing their catalytic activity. Catalyst activities in toluene combustion were evaluated by conversion versus temperature (light-off curves) and conversion versus time catalytic tests. In the case of catalysts prepared by impregnation, the light-off curves for the total combustion of toluene were shifted to lower temperatures with increasing Pt particle size. This suggests that the reaction was sensitive to the Pt structure within the dispersion range of these catalysts. However, the reverse occurred with catalysts prepared by mixing the precursor in the initial aerogel mixture. Results found could be due to the different surface Pt content of these catalysts as revealed by X-ray photoelectron spectroscopy. This difference was related to the growth of large three-dimensional Pt particles on the surface of the less dispersed catalyst. This means that there is a critical Pt particle size above which the toluene combustion activity decreases with increasing Pt particle size, due to the reduction in active surface sites available for the combustion reaction. Other effect that might influence the activity of these last catalysts is the encapsulation of some Pt particles by the carbon matrix. (C) 2004 Elsevier B.V. All rights reserved.