Evolving marine diesel emission regulations, cause significant concern regarding reduction of emissions of oxides of nitrogen (NOx). There is therefore considerable interest to develop and validate Selective Catalytic Reduction (SCR) converters for marine diesel NOx emission control. Substrates in marine applications, need to be robust to survive high sulfur contents and offer cost and pressure drop benefits. Higher cell densities in principle offer benefits with respect to system volume and provide increased catalyst area (in direct tradeoff with increased pressure drop). However higher cell densities may become more easily plugged by deposition of soot and/or sulphate particulate, on the face of the monolithic converter, as well as on its channel walls and catalyst coating, eventually leading to flow restriction. To assess the risks of occurrence of and potential mitigation measures for such phenomena, we have conducted an experimental program and present our findings in order to better understand the relevant phenomena and improve the design capabilities of marine SCR systems.