Uncertainties in cleanup times for soil vapor extraction systems are estimated using mass removal curves generated through Monte Carlo analyses. The primary source of uncertainty is assumed to be soil permeability. Four general model components are coupled to quantify uncertainties: (1) a geological uncertainty model, (2) a spill distribution model, (3) a vapor flow model, and (4) a mass removal model. Spatial variability in permeability has a more significant effect on cleanup time than does overall average permeability because of the effects of channeling. For the examples considered in this study, field measurements of air conductivity have a relatively small effect on uncertainties in cleanup time. The examples also suggest that surface covers can significantly reduce cleanup time in cases where the extraction wells are not immediately within the zone of contamination and for relatively large spills. If wells are completed within the contaminated zone, however, these covers can actually increase average cleanup times. ¿ 2000 American Geophysical Union