Abstract Adsorption of 2,4- d amine, atrazine, terbacil and methyl parathion pesticides on Webster, Cecil and Eustis soils was measured at pesticide solution concentrations ranging from zero to the aqueous solubility limit of each pesticide. Measured equilibrium adsorption isotherms for nearly all soil-pesticide combinations were of nonlinear Freundlich type. The Freundlich adsorption constant ( K) based on soil organic carbon was much less variable for a given pesticide among the four soils than was the K based on total soil mass. The influence of the shape of the adsorption isotherm on the movement of 2,4- d amine and atrazine through water-saturated soil columns was also examined. Pesticide effluent concentrations from soil columns were measured at two input solution concentrations (50 and 5000 μg ml −1 for 2,4- d amine; 5 and 50μg ml −1 for atrazine). In all cases, pesticide mobility was significantly greater for the higher concentrations. Thus, serious errors may be introduced by assuming a linear adsorption isotherm (i.e. pesticide mobility is invariant with input concentration) when predicting pesticide transport from waste disposal sites where high concentrations exist.