All vehicles sold today are required to meet emissions standards based on specific driving cycles. Emissions standards are getting tighter and the introduction of real driving tests is imminent, potentially calling for improved aftertreatment systems. A dual stage catalyst system, with exhaust temperature control, can provide a robust solution to meet challenging modes of operation such as rapid acceleration and other heavy-duty transients. The Ultera® technology, developed and successfully implemented on stationary natural gas CHP (Combined Heat and Power) engines, introduces a second stage catalyst downstream of a three-way catalyst. Air is injected between the two stages to provide oxygen required for the second stage reaction that removes additional CO and NMOG. Critical to the process is to avoid the reformation of NOx. This is achieved by cooling the exhaust gas prior to the second stage, to a temperature range in which CO and NMOG oxidation is extremely effective, while no new NOx is created. The objective of this research was to apply this technology to vehicle engines, with the primary interest being gasoline-fuel, direct fuel injection, and more dramatic transient loading. Testing of a ULEV compliant light duty truck (LDT) and a European passenger vehicle was conducted using a chassis dynamometer. Optimization of control temperature and air- injection flow was studied. Also examined were customized catalyst formulations for enhanced hydrocarbon reduction. Results showed significant reductions of CO and NMOG, with no negative impact on NOx. There was also no measurable impact on fuel economy, but further study is required to include the parasitic loads of cooling and air injection in order to fully quantify the impact to mpg. Future development work can provide opportunities to further reduce NOx emissions through chemistry and integration with engine operation.