The state and dispersion of a highly dispersed Rh/Al/sub 2/O/sub 3/ catalyst in various reactive atmospheres were investigated by low-temperature infrared spectroscopy of chemisorbed CO. Rh metal particles disintegrate in the presence of CO, and Rh/sup +/ species form as a result of oxidation by surface -OH groups at temperatures less than or equal to 300 K. The predominant oxidation process at higher temperatures is presumably related to the dissociative chemisorption of CO. The rates of these processes are strongly dependent on the initial reduction conditions, being less for the more severely reduced samples containing the larger Rh metal particles. A cyclic process of CO adsorption and H/sub 2/ reduction leads to particle growth and increasing resistance of the larger particles to disintegration and oxidation in CO. Simultaneous exposure of the samples to CO and H/sub 2/ does not prevent the oxidative conversion of some of the Rh/sup 0/ into Rh/sup +/ at temperatures greater than or equal to 473 K, but complete conversion of the Rh/sup 0/ is hindered because the CO ligands are removed by reaction with the H/sub 2/. Water vapor in the presence of CO assists the formation of Rh/sup +/ species.