We present transient kinetic data that demonstrate an inhibiting effect of ammonia on the NH3-selective catalytic reduction (SCR) of nitric oxide (NO) at low temperatures over a V2O5-WO3/TiO2 commercial catalyst for vehicles. This effect has significant mechanistic as well as practical implications. It cannot be reproduced by conventional or modified Eley-Rideal approaches used in the past for SCR-deNOx stationary applications, but is well described by a novel dual-site redox rate expression assuming that ammonia may block the vanadium-related catalyst sites for NO + NH3 activation. The same rate model also successfully represents the kinetic influence of the oxygen concentration. It is shown that account of ammonia inhibition at low temperatures is critical for simulating the highly transient operation of onboard SCR converters for diesel exhaust aftertreatment. (c) 2006 American Institute of Chemical Engineers.