In situ Fourier transform infrared spectroscopy was employed to investigate the formation of CO and other adsorbed species on Al2O3-supported catalysts exposed to a supercritical mixture of CO2 and H-2 (P = 138 bar; T = 342 K; molar CO2/H-2 = 19). On just the Al2O3 support, surface carbonates are observed consistent with literature reports with some evidence of surface formates. On Pd/Al2O3, however, several surface species including carbonates, formates, and CO are unambiguously observed. On Pd/Al2O3, a peak at 1900-1920cm(-1) corresponds to adsorbed CO. Based on the presence of water peaks at reaction conditions, the reverse water gas shift reaction is the most plausible mechanism for CO formation. The CO peak evolves with time on stream, gradually increasing in intensity from similar to 20 min to 5 h. This suggests that short-residence time continuous reactors are preferred over batch reactors to minimize the effects of possible catalyst deactivation by CO. Interestingly, the CO peak was not observed on either Ru/Al2O3 or Ni/Al2O3 catalysts suggesting that either of these catalysts may be better suited than Pt- or Pd-based catalysts for hydrogenation in supercritical CO2. (c) 2007 Elsevier Ltd. All rights reserved.