A model has been developed for prediction of the reversible washout of SO2 emitted from power plant plumes and other sources. Predictions of this computer code model compare favorably with washout measurements made during a number of controlled source experiments and four power plant experiments. An application of the model to previous experimental conditions of high background rain acidity shows that 'negative washout' can occur as a result of desorption of SO2 from the rain below the SO2 plume. Scavenging rates (cross-plume integrated fluxes) measured during the power plant study show that the rate of deposition of sulfate was 1--5 times greater than that of SO2 at distances from 0.4 to 11 km from the stack. The experimental results were applied to two very simple SO2 oxidation models: one involving liquid phase (raindrop) oxidation and one considering 'in-plume' oxidation. Either mechanism appears to be compatible with the observations if the SO2-sulfate reaction half time in each case is of the order of 0.07 h. This result follows from a number of assumptions, including a reasonable washout coefficient for sulfate in the in-plume case and equilibrium SO2 washout in the liquid phase case.