A mathematical model for soil vapor extraction (SVE) is developed which models mass transport of volatile organic compounds (VOCs) through aqueous boundary layers by means of a distributed diffusion approach. The well configuration modeled is that of a buried horizontal slotted pipe. The model yields high off-gas VOC concentrations initially, followed by a rapid drop-off to a relatively long plateau, followed in turn by a terminal region of tailing, the length of which is highly variable and is determined by the range of the distribution of values of the aqueous diffusion layers. The results suggest that it will be difficult, if not impossible, to develop models which permit the accurate prediction of SVE cleanup times from data taken in short-term pilot-scale experiments during which only 5-25% of the VOC present in the domain of influence of the well is removed.