Temperature-programmed reduction and oxidation (TPR and TPO) have been used to study the state of cobalt and rhodium in a series of Co---Rh/¿-Al2O3 catalysts. The results show that rhodium enhances the reducibility of part of the cobalt, but that it does not prevent the formation of cobalt aluminate, which is irreducible below 773 K. TPR of the coimpregnated Co---Rh/¿-Al2O3 catalyst shows a reduction peak at a much lower temperature than that of Co/Al2O3. This and the slight shift relative to the peak of Rh/Al2O3 indicates that cobalt and rhodium ions are not far apart after coimpregnation, which explains the easy formation of bimetallic particles during reduction. Passivation (oxidation at room temperature) of the reduced bimetallic catalyst leaves the structure of the bimetallic particles largely intact, but cobalt is oxidized to a great extent while rhodium remains metallic. Passivated Co-Rh particles thus consist of a rhodium kernel covered by cobalt oxide. TPR of passivated catalysts also suggests that already in the reduced state the bimetallic particles are surface-enriched in cobalt. A thorough oxidation of the bimetallic catalysts, on the other hand, leads to a restructuring i.e., the formation of metal oxide particles which are in close proximity.