The sorption of tetrachloroethylene and trichlorobenzene by a subsurface soil that exhibits moderately nonlinear sorption isotherms for each solute was found to be strongly competitive, while that by a soil for which both solutes exhibit more linear sorption isotherms was essentially additive. The isotherm nonlinearity and competitive sorption observed in the former case suggest that a greater fraction of the sorption by that soil occurred on or within tightly ordered or condensed organic matrices rather than within more amorphous soil organic matter. Ideal adsorbed solution theory was used in conjunction with a Freundlich isotherm in a dual-domain model to characterize mathematically the sorption behavior of these two soils in column-type reactor systems. Model parameters obtained independently from breakthrough curves for single-solute systems were used to predict breakthrough curves for bisolute systems. The results demonstrate that (1) even moderate isotherm nonlinearity can affect breakthrough curve behavior in single-solute systems, and (2) the effects of isotherm nonlinearity on solute breakthrough are significantly increased by the additional manifestation of competitive effects in bisolute systems. ¿ American Geophysical Union 1996