This paper reports observations of a tropospheric chemical equator in the Western Pacific region during the Austral monsoon season, separating the polluted Northern Hemisphere from the cleaner Southern Hemisphere. Measurements of carbon monoxide, ozone, aerosol size/composition, and non-methane hydrocarbons were made from aircraft, flying north from Darwin, Australia as part of the Aerosol and Chemical Transport In tropical conVEction (ACTIVE) campaign. A chemical equator, defined as a sharp gradient in the chemical background, was found not to be coincident with the Intertropical Convergence Zone during this period. A pronounced interfacial region was identified between 8.5 and 10 degrees S, where tracer mixing ratios increased rapidly within the boundary layer, e.g. CO from 40 ppbv to 160 ppbv within 0.5 degrees latitude (50 km), with inhibited inter-hemispheric mixing. These measurements are discussed in context using a combination of meteorological and Earth-observing satellite imagery, back trajectory analysis and chemical model data with the conclusion that air flowing into and subsequently uplifted by the active convection of the Tropical Warm Pool (TWP) region in the Western Pacific is likely to be highly polluted, and will perturb the composition of the Tropical Tropopause Layer. The main source of CO and other pollutants within the TWP region is expected to be biomass burning, with extensive fires in North Sumatra and Thailand during this period. The sharp gradient in composition at the chemical equator seen here results from extensive burning to the north, contrasting with pristine maritime air advected from the Southern Indian Ocean by a strong land-based cyclone over the Northern Territory of Australia. ; Hamilton, Jacqueline F. Allen, Grant Watson, Nicola M. Lee, James D. Saxton, Julie E. Lewis, Alastair C. Vaughan, Geraint Bower, Keith N. Flynn, Michael J. Crosier, Jonathan Carver, Glenn D. Harris, Neil R. P. Parker, Robert J. Remedios, John J. Richards, Nigel A. D. NERC The authors wich to acknowledge everyone involved in the ACTIVE project, in particular the pilots and staff of the Airborne Research and Survey Facility. We thant NERC for funding the ACTIVE project. Thanks also go the Australian Bureau of Meteorology and in particular Peter May, for help during the campaign and for access to meteorological data. MLS data was provided by Jonathon Jian at the Jet Propulsion Laboratory (JPL), California. Thanks also go to the TES sceince team at JPL. Fire count data was obtained from the World Fire Atlas project, the Data User Element of the European Space Agency, and plotted by Manasvi Panchal. We also thank the RAAF base, Darwin, for hosting the aircraft and campaign base, and for their logistical support. 31 AMER GEOPHYSICAL UNION WASHINGTON 368CF