The first generation of models ~4-~5'~8-2° intended to simulate SVE transport of VOCs were based upon the assumptions of ideal transport, meaning that the porous medium is assumed spatially homogeneous at the macroscopic scale and that instantaneous equilibrium is established among all phases (i.e. a sorbed, an aqueous, a mobile ideal-gas and possibly an external NAPL phase). 2~ With these assumptions, local equilibrium theory (LET) applies to sorption described by a soil sorption coefficient, and a Henry's Law expression (or equivalent) can be assumed to describe contaminant species concentration in the gas and water phases. Where present, the vaporliquid equilibrium between the NAPL and gas phases is handled by an ideal thermodynamic expression. The advantage of these models is that numerical predictions can often be accomplished by use of a hand-held calculator or a short computer program. But while easy to apply, the models yield poor predictions since (i) the heterogeneity of the vadose zone is well known, and (ii) the assumption of instantaneous sorption between all phases is contradicted by observation. 2t