Sorption of organic vapors on soil increases dramatically as soil-water content decreases in a dry region. Equations for calculating organic vapor partition coefficients in unsaturated soils as a function of soil-water content are proposed. The equations were based on the hypothesis that organic vapor in soils are found adsorbed onto water-solid and air-solid interfaces and dissolved in the soil solution. In the dry range, where water in soils can be considered the sorbate, water vapor and organic vapor compete for sorption sites with water vapor adsorbed preferentially at the air-solid interfaces because of the higher polarity of water molecules. The air-solid interfaces that are not covered by water molecules and are available for sorption of organic vapor can be estimated according to the Brunauer-Emmett-Teller adsorption theory. The predictions made by the proposed equations were compared with partition coefficients of three volatile organic compounds (VOCs)-benzene, toluene, and trichloroethylene-and a pesticide, diazinon (O,OdiethylO-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate) in Yolo silt loam, and with partition coefficients of the same three VOCs for two clay minerals. The measured and predicted partition coefficients agreed reasonably well. All parameters of the proposed equations are measurable, and no curve-fitting is needed.