Competitive adsorption of carbon tetrachloride, chloroform, and 1,1,1-trichloroethane from aqueous solutions onto soils was investigated. The experimental data were measured through batch equilibration studies in conjunction with gas chromatography/mass spectrometer analysis. Equilibrium isotherms were generated on three soil samples with different compositions. Competitive effects were analyzed by comparing single-component isotherms to binary and ternary solute isotherms and were quantified in terms of selectivity of an individual compound with respect to another constituent in the mixture. Influence of cosolvent was examined by generating single and multiple solute sorption isotherms from 25% (volume) and 50% (volume) methanol-water solutions. Isotherm linearity was observed, even in the vicinity of saturation, indicating constant retardation rate during the subsurface transport. The order of equilibrium uptake capacity of different soils was consistent with the order of their specific surface areas as determined through nitrogen adsorption. Competitive sorption from multisolute solutions was within a selectivity range of 1.17-2.22. While the contaminant properties determined the order of competition, soil properties determined its quantification. An opposite relationship between the organic matter content and the selectivity for the compound with higher polarity was observed. In the presence of cosolvent, soil＇s equilibrium capacity significantly decreased as the volume fraction of methanol increased. Furthermore, competition among the solutes decreased, exhibiting a selectivity range from 1.01 to 2.07. High adsorption tendency of the cosolvent on soil and increased solubility of the contaminant species in the mixed solvent are the dominant factors for the variations.