The time taken for an exhaust emission-control catalyst to reach its operating temperature for hydrocarbon oxidation is a major barrier to achieving ultra-low emissions from vehicles. A new approach for achieving rapid catalyst light-off from a vehicle cold-start has been devised and demonstrated on an automobile. An important component in the system is a Pd-Pt-based catalyst which, under net lean conditions in an exhaust stream containing high levels of CO, is active at ambient temperature for the highly exothermic CO oxidation reaction. The Pd-Pt catalyst has positive-order kinetics with respect to CO for the CO oxidation reaction; hence, increasing the level of CO in the feed leads to increasing reaction rates and a faster temperature rise for the catalyst. In practice, this means that enriching the air to fuel mixture supplied to the engine at cold-start (with a secondary air source to provide at least the required amount of oxygen for complete CO conversion in the exhaust) enables the catalyst to reach operating temperature within seconds of starting the engine. In the present work, this ambient temperature catalyst is combined with an upstream water trap (zeolite 5 Angstrom) and hydrocarbon trap (zeolites H-ZSM-5 or H-Beta). The traps delay the exposure of the catalyst to these potentially inhibiting species until it has reached a temperature at which it can effectively combust hydrocarbons. When tested fresh, this system demonstrated high levels of hydrocarbon conversion throughout the start-up phase of a Federal Test Procedure cycle.