In order to study the mechanism of aerosol sulfate formation, weekly samples of SO2 and aerosol SO=4 were collected at Hubbard Brook Experimental Forest, West Thornton, New Hampshire from July to December, 1980. Samples were analyzed for concentration and sulfur isotopes (Δ34S). Late summer-early fall samples are characterized by high, variable SO=4 levels and low SO2 levels, while late fall-early winter samples exhibit low SO=4 and high SO2 levels. These trends suggest that the oxidation rate varies seasonally, with faster oxidation during warmer months. The sulfur isotopic fractionation between aerosol SO=4 and SO2 is intermediate between that expected from homogeneous and heterogeneous reactions, indicating that both processes can be important. The isotopic data suggest that homogeneous oxidation reactions are more important than solution reactions, particularly during warm months.