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Greenhouse gas emissions (GHG) from light-duty vehicles have received attention recently because of increased focus on global warming and climate change. Relative to emissions of regulated pollutants like hydrocarbons and nitrogen oxides, nitrous oxide (N2O) emissions from all vehicles are generally very low. However, N2O is a powerful greenhouse gas, and small emissions of N2O can contribute substantially to total GHG inventories. Two fleets of different vehicle models, both meeting the current US Tier 1 emission standard, were evaluated in an effort to develop a better understanding of N2O emissions from modern three-way catalyst-equipped vehicles. Nine 1997 Ford Crown Victoria vehicles operating on clean-burning US Federal Phase 2 Reformulated Gasolines were assessed over 60,000 miles. For additional comparison, testing was also conducted with catalysts from six 1994 Toyota Camry vehicles, which had previously undergone 110,000 miles of controlled mileage accumulation.