As part of the Indian Ocean Experiment (INDOEX), measurements of CH2O were carried out with a tunable diode laser absorption spectrometer on board the National Oceanic and Atmospheric Administration research vessel Ronald H. Brown during spring 1999. The average CH2O concentration measured in pristine southern hemispheric air over the Indian Ocean (IO) was 210 parts per trillion by volume (pptv). CH2O levels measured in the Northern Hemisphere show a strong dependence upon the air mass origin. North of the Intertropical Convergence Zone a mean concentration of 430 pptv was found in surface air over the IO. When the origin of the air masses did not change during the day and the air was not influenced by anthropogenic emissions, a clear diurnal cycle was observed with a minimum in the late morning and a maximum in the late afternoon. The amplitude of the mean diurnal pattern is around 150 pptv in the Southern Hemisphere and around 200 pptv in the Northern Hemisphere. In polluted air masses of the Northern Hemisphere, CH2O is positively correlated with CO. The ratio CH2O/CO is 3.2¿10-3 indicating well aged air with only a reduced amount of CH2O precursors left. To date, the reason for the observed correlation remains open to speculation. ¿ 2001 American Geophysical Union