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Two-brick (tandem) three-way underfloor catalyst systems provide greater emission reduction performance compared to comparable single brick TWC systems, which contain the same amount of platinum group metal (PGM) for the same catalyst volume. This superior emission reduction performance is speculated to be due to front catalyst activity promoted by heat transfer from reverse exhaust gas flow in the gap between the front and the rear catalyst of tandem TWC system (hereinafter, tandem gap). Furthermore, the following findings were obtained by conducting experiments with model catalysts. 1) During catalyst light-off phase, conversion efficiency strongly depends on activity of the front portion of catalyst where temperature rises rapidly.