A tracer technique for following the SO4 formation from SO2 oxidation in ambient air is reported. Like SO2, Se is emitted to the atmosphere during coal combustion. Both SO4- and Se-bearing aerosols show similar physical behavior; Se, however, is not formed in the atmosphere. We have used this difference to determine in situ SO4 formation from SO2 oxidation. The technique uses SO4 and Se concentrations measured at two points of an air mass system. Results from samples collected at Mayville, and 530 km downwind at Whiteface Mountain, New York, are reported. Air trajectories were used to isolate two time periods (one each during the summers of 1981 and 1983) when air masses traveled from the high-SO2 emission region in the Midwest to Mayville and then to Whiteface Mountain. Both periods were accompanied by hot and humid conditions across the Northeast and SO4 concentrations of the order of 30 μg/m3. An average SO4 production rate of 1.0 μm/m3/h was obtained. A simple model based on the SO4/Se ratios at a series of sites suggests that the SO2 gas phase oxidation rate was ~3% per hour. ¿ American Geophysical Union 1992