A NOx self-inhibiting effect was observed in the selective catalytic reduction (SCR) of NOx on a diesel engine over a Cu-zeolite catalyst with NH3 as the reductant (supplied either directly or as urea). The effect was strongest at low-temperatures (<250 degreesC) and became negligible above 300 degreesC. NOx self-inhibition was observed both for NO and NO/NO2 mixtures, and with both low- and high-sulfur (S) fuel. Based on reported mechanisms in the literature, the NOx self-inhibition is attributed to competitive adsorption between ammonia and NOx at the active Cu sites, with NOx dominating at low-temperatures and/or high concentrations. Hence, engine operating strategies that allow NH3 to compete successfully with NOx (e.g. exposing the catalyst to urea under conditions of low NO, coverage-namely near-zero-torque conditions where engine-out NOx concentrations are negligible, or high-temperature conditions where NOx coverage is low) result in enhanced transient NOx conversion under subsequent low-temperature conditions. In particular, our findings are consistent with results of Centi et al. on CuO/Al2O3 [J. Catal. 152 (1995) 75; J. Catal. 152 (1995) 93] indicating the formation of strongly-adsorbed nitrate species that block the reactivity of Cu sites.