This study was conducted to investigate the relative effects of physical heterogeneity, gas-liquid mass transfer, and rate-limited sorption on the gas-phase transport of contaminants in idealized unsaturated homogeneous and heterogeneous porous media. The transport of methane in the unsaturated homogeneous porous medium was ideal, whereas that of trichloroethene and benzene was nonideal, governed by rate-limited diffusive mass transfer in immobile water and by rate-limited sorption/desorption. Transport of both methane and trichloroethene through the unsaturated heterogeneous porous medium was nonideal. Gas-phase mass transfer between unsaturated advective and nonadvective domains caused nonideal transport of methane. Trichloroethene nonideality was due to a combination of gas-phase mass transfer between advective and nonadvective domains, diffusion within immobile water, and rate-limited sorption/desorption. The transport of trichloroethene through the heterogeneous porous medium was predicted by use of a multiprocess mass transfer model, wherein all parameters were estimated independently. ¿ 1998 American Geophysical Union