A one-dimensional numerical model is presented for simulating the enhanced biorestoration of aquifer material contaminated with residual nonaqueous phase liquid (NAPL) hydrocarbon. The model simulates the simultaneous dissolution, transport, and biodegradation of individual hydrocarbons under oxygen-limiting conditions. Partitioning between the NAPL and aqueous phase is modeled as a linear first-order process where the NAPL is composed of two fractions, a fast oil fraction with a high mass transfer rate and a slow oil fraction with a much lower mass transfer rate. Microorganisms are assumed to be present attached to the soil grains and are assumed to be immobile. Biodegradation of the problem compounds is simulated as a multistep process where the parent compounds are first biotransformed to oxygenated intermediates. These intermediates may then be mineralized to CO2 and H2O. Comparison of model simulations with experimental results from soil columns containing residual hydrocarbon and an active microbial population indicate that the model is capable of simulating the extent of hydrocarbon biodegradation using reasonable model parameters. Model simulations indicate that if benzene, toluene, and xylene are assumed to be completely mineralized, the model will greatly underestimate the extent of biotransformation. The model overestimated the extent of hydrocarbon biotransformation at low substrate concentrations. ¿ American Geophysical Union 1993