Before 1991, Baehr et al. [1989] had measured liquid-vapor mass-transfer rates for gasoline in homogeneous media. Even for a fairly high air-phase velocity, the experiments were matched well by an equilibrium transport model. It was indicated that contact of the flowing air phase with the contaminant would be important in a heterogeneous medium. The authors speculated that equilibrium might not hold for a pure liquid in place of gasoline. Comparing simulations to laboratory experiments, Gierke et al. [1992] found that vapor extraction was affected by nonequilibrium transport in moist soils in which soil particles and immobile water were aggregated. The nonequilibrium effects were attributed to diffusion within aggregates, a more complicated mechanism than first-order kinetic mass transfer that has not been included in models. Another complication in mass transfer was reported by Brusseau [1992b], observing that the sorption rate constant depended on gas velocity, so that the validity of LEA could not be simply pegged to the reciprocal of velocity. Cho et al. [1993] measured gas-water transfer rates in the laboratory and the field for low gas-flow velocities in the unsaturated zone. They found that simulation of the dynamics of VOCs during infiltration required kinetic transfer. As with NAPL dissolution, the range of validity of LEA for VOC mass transfer is uncertain at this time, but it appears that nonequilibrium phenomena will be important in at least some applications.