Rhodium catalysts, supported on six gamma-Al2O3 supports with different crystallinities, were exposed to sequential treatments in hydrogen at 500 degrees C, in oxygen at 760 degrees C, in hydrogen at 500 degrees C and at 760 degrees C, respectively. Samples were characterized by X-ray diffraction and hydrogen chemisorption at various stages in the sequential treatment. Based on the characterization results, it is concluded that the formation of crystalline Rh2O3 is a function of gamma-Al2O3 crystallinity; formation of crystalline Rh2O3 increased with increasing crystallinity of gamma-Al2O3 during treatment in oxygen at 760 degrees C. The crystalline Rh2O3 formed during treatment in oxygen at 760 degrees C was reduced to Rh metal by hydrogen at 500 degrees C, but most of the Rh did not adsorb hydrogen at room temperature. Subsequent treatment in hydrogen at 760 degrees C increased the hydrogen adsorption capacity by as much as a factor of three. X-ray line broadening measurements showed that oxygen treatment of reduced Rh/gamma-Al2O3 at 760 degrees C followed by hydrogen reduction at 500 degrees C resulted in significant increases in Rh crystal size; further treatment in hydrogen at 760 degrees C resulted in additional sintering of Rh.