The chemical interactions between CO and H2 over rhodium were investigated for H2/CO/O2/N2 mixtures with H2:CO volumetric ratios 1:5–3:1, overall fuel-lean equivalence ratios φ = 0.13 and 0.23, a pressure of 5 bar, and surface temperatures 510-610 K. This temperature range was particularly important for cat- alytic 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. Compar- isons of simulations and measurements assessed the performance of the catalytic reaction mechanism for the oxidation of pure CO, pure H2 and H2/CO fuel blends. Transition temperatures were identified below (above) which H2 inhibited (promoted) the oxidation of CO. For a given equivalence ratio, the transition temperatures decreased significantly with increasing H2 :CO volumetric ratio (595 K for H2:CO = 1:5 and less than 535 K for H2:CO = 3:1, at φ = 0.13) while for a given H2 :CO volumetric ratio they dropped moderately with decreasing φ. This behavior was fundamentally different to that of platinum catalysts, whereby transition temperatures depended weakly on H2:CO volumetric ratio and stronger on equivalence ratio. The strong dependence of the transition temperatures on H2: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 H2 on CO oxidation above the transition temperatures was a result of the increased importance of the indirect CO oxidation route via surface COOH.