A numerical model is developed to predict the aqueous concentrations of sparingly soluble compounds resulting from oil, fuel, or chemical spills onto rivers. The model computes the concentration of compounds both in the slick phase and in the aqueous phase by simulating the processes that affect the fate of the spilled compound. Processes simulated by the model include spreading and drifting of the surface slick, evaporation from the slick, dissolution from the slick into the water, volatilization from the water, and longitudinal dispersion in the river. The model is used to simulate a hypothetical spill of jet fuel, demonstrating that the concentration of a compound in the aqueous phase is strongly linked to its concentration in the slick phase. The most soluble and most volatile compounds exhibit the highest aqueous concentrations in the early stages of the spill, but ultimately the less soluble and less volatile compounds reach the highest aqueous concentrations. Streamwise concentration gradients in the slick due to the rapid evaporation of the more volatile compounds are shown to have an effect on the aqueous concentration.