Reduced centres are formed at the silica surface during the pretreatment of Rh/SiO2 catalysts at high temperatures (500-900 degrees C). These centres can be titrated with steam according to the reaction : reduced centre + H2O --> oxidized centre + H-2. The selective formation of H-2 when catalysts pretreated in D-2 are reacted with H2O confirms that steam is decomposed during the titration reaction. XRD, TEM and EDX were used to investigate the morphological and structural changes in the catalysts. Owing to the formation of Rh2Si alloy, the number of reduced centres formed on SiO2 is much greater than the number found with Rh/Al2O3 reduced under similar conditions.