The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al2O3 and Rh/SiO2 catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh3+ species. This behavior is not observed (as expected) for Rh/SiO2 catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed.