The effect of phosphorus (P) on iron (Fe)-zeolite based selective catalytic reduction (SCR) monolith catalysts was studied in the absence and the presence of sulfur, and at two different exposure temperatures (220 and 450 8C). It was found that exposure of Fe-zeolite SCR catalyst to P resulted in a loss of NOx conversion and an increase of NH 3 slip relative to a degreened baseline sample. Some of the increased slip was attributed to a loss of NH 3 storage capacity due to P blockage of adsorption sites. Additional increased slip resulted from a rise in the slip to storage ratio, likely due to a shorter effective channel length for NH 3 adsorption since most of the P blocked sites at the sample inlet. DeNOx activity was then reduced due to the decreased amount of adsorbed NH 3 . P exposure using zero sulfur fuel resulted in the same amount of deactivation as aging done using low sulfur (350 ppm S) fuel, suggesting that this amount of S did not affect this formulation. An initial loss of activity proved to be due to hydrocarbon exposure, and was reversible simply by ramping the sample up in temperature during the evaluation test. Increasing the exposure temperatures also increased the loss of DeNOx performance. It is proposed that P impacts these catalysts by physically blocking adsorption sites for NH 3 , primarily at the monolith channel inlet.