Much attention has been paid to the effect different catalyst formulations have on conversion efficiency. However, the role of the catalyst support structure has not been investigated. The purpose of this study is to investigate the effect that the catalyst support structure has on gas phase conversion efficiency. Understanding this effect is important in the assessment of the practicality of 4-way diesel exhaust emission control based on current, commercially available, technologies.This investigation begins with an analysis of the mass transfer properties of flow-through and wall-flow style honeycomb materials. Next, chemical reaction kinetics are discussed and theoretical models are developed for both structures. Results of the analysis presented in this paper conclude that flow-through type structures have better conversion efficiency than wall-flow style supports due to increased reaction residence time. This increased reaction residence time is attributed to differences in the mass transfer characteristics of the two structures. These conclusions were validated via experiments that used both bottled gases and diesel engine exhaust.The results of this study suggest that attainment of a space efficient 4-way diesel exhaust gas emission control device will require major changes in the filter media and/or development of improvements in the catalyst coating of filter media.