Vapor sorption isotherms on dry Woodburn soil at 20-30C were determined for benzene, chlorobenzene, p-dichlorobenzene, m-dichlorobenzene, 1,2,4-trichlorobenzene, and water as single vapors and for benzene, m-dichlorobenzene, and 1,2,4-trichlorobenzene as functions of relative humidity (RH). Isotherms for all compounds on dry soil samples are distinctively nonlinear, with water showing the greatest capacity. Water vapor sharply reduced the sorption capacities of organic compounds with the dry soil; on water-saturated soil, the reduction was about 2 orders of magnitude. The markedly higher sorption on organic vapors at subsaturation humidities is attributed to adsorption on the mineral matter, which predominates over the simultaneous uptake by partition into the organic matter. At about 90% RH, the sorption capacities of organic compounds become comparable to those in aqueous systems. The effect of humidity is attributed to adsorptive displacement by water of organics adsorbed on the mineral matter. (Copyright (c) 1985 American Chemical Society.)