The chemical interactions between CO and H-2 over rhodium were investigated for H-2/CO/O-2/N-2 mixtures with H-2:CO volumetric ratios 1:5-3:1, overall fuel-lean equivalence ratios phi = 0.13 and 0.23, a pressure of 5 bar, and surface temperatures 510-610 K. This temperature range was particularly important for catalytic ignition in hybrid hetero-/homogeneous combustion concepts of large gas-turbines operating at part-load or idling conditions and in recuperative micro-turbine-based microreactors at normal operation. In situ Raman measurements of major gas-phase species concentrations were carried out over the catalyst boundary layer, while 2-D simulations were performed with a detailed catalytic reaction scheme. Comparisons of simulations and measurements assessed the performance of the catalytic reaction mechanism for the oxidation of pure CO, pure H-2 and H-2/CO fuel blends. Transition temperatures were identified below (above) which H-2 inhibited (promoted) the oxidation of CO. For a given equivalence ratio, the transition temperatures decreased significantly with increasing H-2:CO volumetric ratio (595 K for H-2:CO = 1:5 and less than 535K for H-2:CO = 3:1, at phi = 0.13) while for a given H-2:CO volumetric ratio they dropped moderately with decreasing phi. This behavior was fundamentally different to that of platinum catalysts, whereby transition temperatures depended weakly on H-2:CO volumetric ratio and stronger on equivalence ratio. The strong dependence of the transition temperatures on H-2:CO volumetric ratio over rhodium pointed to the advantage of this catalyst when used for high-hydrogen-content (> 80% volume) fuels in power generation applications. The promotion effect of H-2 on CO oxidation above the transition temperatures was a result of the increased importance of the indirect CO oxidation route via surface COOH. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.