Steady-state tracing techniques have been applied to investigate mechanistic aspects of the CH4 reforming reaction over CO2 over Ph supported on yttria-stabilized zirconia (YSZ) and Al2O3 as catalysts. It was found that the surface coverage of active carbon-containing species, which are found in the reaction pathway to CO formation, is of the order of 0.2 over the Rh/Al2O3 catalyst, while it is very small (theta(c) < 0.02) over Rh/YSZ. The surface coverage of active oxygen-containing species which lead to the formation of CO is found to be very small over both Rh/Al2O3 and Rh/YSZ catalysts. However, over the R/YSZ catalyst it was found that there exists a large reservoir of lattice oxygen species of the carrier which interact reversibly with gaseous CO2 under reforming reaction conditions. A spillover of these lattice oxygen species onto the Rh surface seems to occur, contributing to the formation of CO and H2O. This reaction route proceeds in parallel with the reforming reaction on the Rh surface.