During the Indian Ocean Experiment (INDOEX) in February-March 1999 the impact of continental outflow to the Indian Ocean was analyzed. On board the R/V Ronald Brown altogether 93 air samples were taken for analysis of nonmethane hydrocarbons (NMHC), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), and the 14C/12C and 18O/16O isotope ratios of CO. Five types of air masses differing in origin, degree of pollution, and chemical age were identified based on back trajectory analyses and the trace gas data, supported by continuous CO and ozone (O3) observations from other investigators. The Indian Ocean was found to be frequently affected by nearby emissions from the Indian subcontinent and Indochina, but the strongest pollution event (characterized inter alia by high mixing ratios of medium- and long-lived NMHC) was due to long-range advection from the extratropical northern hemisphere. Carbon monoxide 14 showing a distinct meridional profile unequivocally confirms this remote impact. The ratio acetylene/CO was found to be often inadequate as a measure for atmospheric processing, the integrated influence of OH chemistry and mixing. Our data suggest that the influence from fresh continental pollution was less pronounced along the INDOEX R/V Ronald Brown cruise compared to observations made during other tropical campaigns, such as the Pacific Exploratory Mission-West B in the Pacific Ocean.