The effect of hydrogen on CO-induced topological changes in alumina-supported Rh has been investigated by following the development of the infrared bands due to Rh/sup I/(CO)/sub 2/ and Rh/sub x/-CO species. It was found that the presence of H/sub 2/, even in a large excess, could not prevent the disrupting effect of CO, i.e., the formation of isolated RH/sup I/ sites at 300-373 K. However, above 373 K H/sub 2/ completely suppressed the disruption of Rh-Rh bond. In addition, H/sub 2/ enhances the agglomerizing effect of CO (i.e., re-formation of metallic Rh from isolated Rh/sup I/) exhibited at and above 448 K. It is proposed that in the phenomena observed the formation of rhodium carbonyl hydride, the reducing effect of H/sub 2/, and the enhanced mobility of Rh play a dominant role.