The influence of soil-water ratio (Rs/w) on adsorption-desorption kinetics was shown in a quantitative study with isoxaben, a benzamide herbicide, on a silt loam soil (Versailles soil). Such a study made it possible to describe how equilibrium was reached when Rs/w ranged between 0.05 and 1.60, and underlined the interest of the three mathematical models used (hyperbolic model, and two-and three-compartment models). For each model the parameters (especially k21 and k12, the adsorption and desorption rate constants, respectively) were estimated by nonlinear regression. The most suitable model was obtained when the soil was considered as two compartments intended as the fast adsorption phase and the slow adsorption phase, respectively. The increase in the maximum amount of adsorbed isoxaben (Q_max) was proportionately lower as Rs/w increased. The equilibrium constant (K_e = k21/k12) indicated that isoxaben tended to desorb as Rs/w decreased (Rs/w lower than 0.4). The interest of each mathematical model is discussed.