Partitioning of a species between aqueous solution and sorbed phases is commonly described as an equilibrium process and modeled by a sorption isotherm. These sorption models may be derived from the cumulative effect of many underlying specific reactions. However, in heterogeneous systems, such as soils, the full array of these reactions and the relative contribution of each are unknown. This problem may be circumvented, without sacrificing consistency with the underlying reactions, by use of a sorption isotherm which faithfully describes measured sorption behavior. Since transport predictions based on one or another isotherm model may be quite different, careful choice of an isotherm is critical. The objective of this study was to examine the effect of isotherm type on predicted solute mobility in soil. The derivation of several isotherms from either specific reactions or a combination of ''local'' isotherms with weighting functions is first presented. The use of Freundlich and logarithmic Scatchard plots for selecting the proper sorption isotherm is discussed. Isotherm models, fit to experimental data from the literature, are then used in a transport simulation model. Differences between results are discussed. Also, a brief sensitivity analysis which shows the effects of isotherm parameters on transport behavior is presented. ¿ American Geophysical Union 1994