The efficiency and durability of catalytic converters for automobiles are determined by several heat and mass transport mechanisms acting in concert. This study characterizes these mechanisms with measured temperature and concentration profiles throughout a large-scale catalytic passage at fixed wall temperature. The increased passage size allows the concentration field within the passage to be accurately monitored. A small isokinetic sampling probe and laser positioning system enable the concentrations to be spatially resolved to within 0.04 mm and ten transverse locations are sampled at each axial station. The active walls are coated with a Pt catalyst over a production alumina washcoat containing 28% Ceria on a metal substrate. The walls are 2 cm apart, which is roughly 16 times larger than in a conventional monolith passage, so the Reynolds number is adjusted for scale similarity with commercial devices.A novel temperature control scheme maintains prescribed wall temperatures to within 3°C along the length of the test channel. This tolerance is met even when 85% of the CO is oxidized in an inlet mixture with 4% CO. Concentrations at the surface are inferred from measured concentration profiles in the boundary layer (at several axial positions). Isothermal operation decouples thermal and concentration dependencies of the chemical kinetics at the surface, which allows the development of more accurate rate laws. In this paper, an overall rate law for CO oxidation is reported for temperatures from 180°C to 300°C and CO concentrations from 0.1° to 4°. The rate expression is based on the dual-site Langmuir-Hishelwood mechanism with empirical modifications to account for the effect of CeO2.