Rh/ZrO 2 ±SiO 2 catalysts prepared by impregnation of mixed ZrO 2 ±SiO 2 oxides obtained by using the sol±gel method were characterised and used in the combustion of CH 4 . It was found that the speci®c area and porosity depend strongly on the pH of gelation. Two different Zr precursors were used: Zr(acac) 4 and Zr alkoxide. When the former precursor is used, no signi®cant changes in the surface area are detected, however, important changes in pore size distribution are observed. In those samples prepared from Zr alkoxide, both the surface area and the porosity change signi®cantly. No important changes in rhodium dispersion were observed by TEM, in agreement with XPS results. On the contrary hydrogen chemisorption results indicate that the hydrogen uptake of the sulfated samples drops drastically compared with those in which sulfate ions are absent. This behaviour has been attributed to a poisoning of rhodium particles by sul®de species produced by reduction of sulfate ions, as was revealed by TPR. The catalytic activity in the methane combustion is affected by the presence of chloride ions, which induce an inhibitory effect on the combustion. Additionally, the presence of micropores also decreases the activity due to difusional limitations.