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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.