The validity of steady-state models for steam stripping of volatile and semivolatile organic compounds (VOCs and SVOCs) is explored within the framework of a simple one-compartment approach. The steady-state approximation appears to be valid after a relatively short warming phase. A mathematical model for the in-situ steam or hot-air stripping of volatile and semivolatile compounds from vadose zone soils at hazardous waste sites is presented. The model assumes that the pneumatic permeability of the soil is constant and isotropic, and that the adsorption isotherm of the contaminant on the soil is linear. Diffusion kinetics are included in the model; it is assumed that the contaminant is adsorbed in porous low-permeability lenses of clay, till, or silt, and must diffuse to the surfaces of these structures in order to be removed by the advecting vapor phase. The dependence of results on model parameters is explored.