During the intensive field phase of the Indian Ocean Experiment (INDOEX), measurements of the atmospheric chemical and aerosol composition over the Indian Ocean were performed from a Cessna Citation aircraft. Measurements were performed during February and March 1999 over the northern Indian Ocean from 70 ¿E to 80 ¿E, and from 8 ¿N to 8 ¿S in the 0--13 km altitude range. An overview of the trace gas measurements is presented. In the lowest 3 km the highest levels of pollution were found during February 1999, mostly originating from northeastern India and southeastern Asia. Lower levels of pollution were detected in March 1999, when the sampled air mostly originated from the Arabian Sea region. The mixing ratios of a number of trace compounds, indicative of biomass burning, were well correlated. The pollutant emission factors inferred from the measurements are consistent with literature values for fire plumes, confirming that the residential use of biofuels in Asia is a major source of gaseous pollutants to the atmosphere over the Indian Ocean, in accord with emission databases. The removal of reactive trace gases was studied over an extended area without interfering local emissions, and is shown to be governed by photochemical processes rather than by mixing and deposition. At intermediate altitudes of 3--8 km the mixing ratios of all trace gases other than ozone were generally lower, and the measurements suggest that the photochemical processing of these air masses is much more extensive than in the 0--3 km range. In the 8--13 km altitude range some evidence is obtained for the importance of convective cloud systems in the transport of gaseous pollutants to the upper troposphere. ¿ 2001 American Geophysical Union