Many contaminants of concern in the subsurface are volatile. This fact has been exploited of late in the application of vapor-phase extraction methods for aquifer rehabilitation. The design of vapor-phase extraction systems may be aided by use of mathematical models to simulate the restoration process. Because many common contamination problems are multiphase problems, these simulations often require the use of compositional multiphase models. A common assumption made in compositional multiphase models is that individual solute species are in equilibrium for all phases present in a system: solid, aqueous, immiscible fluid, and vapor. This work reports on an investigation to characterize the rate of mass transfer between the aqueous and vapor phases. A rectangular cross-section experimental apparatus was designed and built to investigate the rate of mass transfer at the interface between the unsaturated and saturated zone. Aqueous solutions with the solute toluene were circulated through a porous media, while depth-averaged aqueous- and vapor-phase concentrations were measured at the inlet and outlet. Experimental control variables included the aqueous-phase velocity, vapor-phase velocity, influent solute concentration, and media size. Data reduction required determination of longitudinal and transverse dispersivity, which was accomplished using a fluoride tracer method along with a two-dimensional finite element model. In addition, a numerical code was developed to model the transport of VOC's in the subsurface, and the code was used in conjunction with the lab data to simulate toluene transport under ambient conditions. Results from the simulations indicate that the aqueous and vapor phases may not be in equilibrium at the unsaturated-saturated zone interface throughout much of the area of a contaminant plume.