Surface O3 levels exceeding 70 nmol/mol, the highest observed to date over the Indian Ocean, were recorded in the Arabian Sea marine boundary layer during the Indian Ocean Experiment (INDOEX) in 1999. Contrasted with this, available O3 observations from nearby coastal cities show generally lower values, rarely exceeding 50 nmol/mol, and averaging 25–35 nmol/mol during daytime, and 5–15 nmol/mol during nighttime. A 3D photochemical model simulates the high marine boundary layer O3 values relatively well, but considerably overestimates the urban O3 values. Surface wind fields show that the air masses with elevated O3 frequently come from India, implying that rapid photochemical production is likely to be occurring in the continental outflow. Furthermore, downwards transport of O3-rich air masses from the free troposphere may also be important in producing some of the observed high-O3 events. Implications of these observations are discussed.