The impact of immiscible-liquid composition on mass transfer between immiscible liquid and vapor phases was evaluated for a complex mixture of chlorinated solvents and petroleum hydrocarbons. A mixture of tetrachloroethene and diesel was discovered at a site in Tucson, Arizona. Partitioning of tetrachloroethene into a layer of diesel has been observed, with resultant concentrations of tetrachloroethene up to approximately 15% by weight. The density, viscosity, surface tension, and interfacial tension were measured for tetrachloroethene-diesel mixtures with tetrachloroethene fractions ranging from 7% to 32%, and the results indicated that immiscible-liquid composition did impact the physical properties of the tetrachloroethene-diesel mixture. The results of batch phase-partitioning experiments were compared to predictions based on Raoult's Law, and the analysis indicated that immiscible-liquid/vapor and immiscible-liquid/water partitioning were both essentially ideal. Flow-cell experiments were conducted to characterize steady-state tetrachloroethene removal from the tetrachloroethene-diesel mixture via vapor extraction. The effluent concentrations for the experiment conducted with free-phase immiscible liquid were comparable to equilibrium values. Conversely, the effluent concentrations were significantly lower for the experiment wherein a residual saturation of immiscible liquid was distributed within sand. The lower concentrations for the latter experiment were attributed to dilution effects associated with a nonuniform distribution of immiscible liquid within the flow cell.