In July and August 1999 a Northeast Oxidant and Particle Study field campaign was conducted in the Philadelphia metropolitan area to determine causes for episodically high levels of O3 and particulate matter ≤2.5 ¿m in aerodynamic diameter. We report emission estimates, weather information, surface O3 monitoring data, and aircraft observations, with a focus on 31 July, the last day of an O3 episode in which concentrations in Philadelphia reached 165 ppb, the highest level observed there in the past 11 years. As is common in the northeastern states, this O3 episode started with the development of a broad ridge over the central United States and ended with the northeast corridor under the influence of an Appalachian lee trough with airflow from the SW in an along-corridor direction. For a portion of the morning of 31 July, winds were nearly stagnant, allowing local emissions to accumulate. In contrast to typical O3 episodes in the northeast, transport on 31 July was limited, and O3 hot spots occurred close to NOx and volatile organic compound (VOC) emission sources. High O3 was observed downwind of Baltimore and Philadelphia, both major urban areas. High O3 was also observed in a less likely region near the Delaware-Pennsylvania border, downwind of Wilmington, Delaware, but near utility and industrial emission sources. Surface O3 monitoring data and morning aircraft observations show that the residual layer aloft contained 80--100 ppb of O3 but almost no O3 precursors. Same-day photochemistry on 31 July caused surface O3 concentrations to increase by 60--80 ppb. Photochemical model calculations indicate O3 production rates in excess of 20 ppb h-1 in regions with NOx > 5 ppb. High NOx concentrations are a consequence of poor ventilation. Peroxide observations and calculations indicate that O3 production is VOC limited in the high-NOx portions of the Philadelphia urban plume. Our results are contrasted against a severe O3 episode that occurred in 1995. While the 1999 episode had stagnation conditions leading to local O3 hot spots, there were mesoscale meteorological features in 1995 that favored interregional transport.