Measurements of ozone and carbon monoxide (CO) from the INDOEX 1999 experiment show large differences in concentrations of ozone and CO between the Northern Hemisphere and the Southern Hemisphere. These measurements confirm the theory that the Intertropical Convergence Zone (ITCZ) serves as a barrier to mixing over the Indian Ocean, effectively separating the polluted air in the Northern Hemisphere from cleaner air in the Southern Hemisphere. In spite of CO levels similar to those observed off the coast of North America, there is relatively little ozone off the coast of India. The ozone-to-CO ratio in air coming from India is 0.14, lower than 0.3 or 0.4 in air being transported to Bermuda from North America. Diurnal cycles are observed in both CO and ozone. INDOEX data taken onboard the R/V Ronald H. Brown show an average diurnal cycle in ozone of 20%, while data from the island of Kaashidhoo in the Republic of Maldives indicate a diurnal variation of 19%, consistent with our analyses of other experiments. Diurnal variations of this magnitude are larger than expected from ozone destruction by conventional HOx chemistry alone, implying that the sink of ozone in the remote marine boundary layer is likely considerably larger than had been assumed. Additional chemical cycles must be fairly substantial-large enough to rival HOx chemistry in ozone destruction.