Sulfur dioxide injected into the atmosphere is most likely oxidized into sulfate. Two major oxidation pathways are possible: 1) a homogeneous pathway involving gas reaction with hydrosyl radicals and 2) a heterogeneous pathway involving aqueous dissolution or aerosol reactions. The relative importance of these reaction pathways conditions is controversial. Sulfur isotope ratios can be used to quantify the relative importance of these reaction pathways. However, its application was severly hampered by the fact that the isotope fractionation factor for the homogeneous pathway was not known (Caron et al., 1986; Nriagu et al., 1987). A significant isotope fractionation in the homogeneous SO2 oxidation is identified for the first time using an ab initio quantum mechanical calculation. By using the sulfur isotope fractionation factors we demonstrate a technique that uses measurements of the sulfur isotope ratio in gaseous SO2, aerosol SO4 and sulfate in wet precipitation to quantify the relative importance of the homogeneous and heterogeneous reaction pathways as well as the in-cloud scavenging of sulfur dioxide for a set of isotopic observations at New Haven, CT, USA. ¿ American Geophysical Union 1994