The capacity of natural bacteria to aerobically degrade hydrocarbon vapors was measured and modeled to assess the potential of bioventing to reduce exhaust vapor treatment requirements at a petroleum spill site. Five sets of aerobic soil microcosms from the vadose zone of a Massachusetts Highway Department contaminated right-of-way were dosed with different initial petroleum vapor standard concentrations, then monitored by gas chromatographic analysis over a 55-day period. The five sets yielded an average maximum reaction rate of 20 μg/m3 (soil gas)-sec, which compared favorably with studies of light hydrocarbon vapor degradation in sandy soils from other sites. The calibrated rate was incorporated into a steady-state bioventing model that simulated the evaporation of 34 000 L of petroleum over a 170-year natural release period and an 8-year accelerated release period for 10-day residence time. Aerobic degradation for a 10-day residence time reduced exhaust vapor concentrations by over 100 percent for natural release rates, with a 13 percent reduction under accelerated conditions.