A technique based on SO4/Se ratio is developed to determine in-cloud formation of SO4 from the oxidation of SO2. Whereas nucleation scavenging is the only significant source of Se in cloud water, both nucleation scavenging and dissolution of SO2 with subsequent oxidation are significant sources of SO4 in cloud water. Based on the assumption that no fractionation occurs between Se and SO4 aerosols during nucleation scavenging, it is shown that a simultaneous measurement of SO4 and Se in cloud water and ambient air permits a direct determination of in-cloud SO4 formation. The data to support this assumption are provided. The results from two cloud event during July and August, 1987 show that in-cloud formation ranged between 8 and 28% in ten hourly samples. ¿ American Geophysical Union 1989