The adsorption-desorption behaviour and the degradation of an ionic herbicide (2,4-D) were examined using 10 soils from New Zealand that differed in their organic matter and clay content. Adsorption isotherms for 2,4-D were adequately described by the Freundlich isotherm and the values of the exponent N of the Freundlich isotherm were close to 1 (0.92-0.98), indicating that the adsorption isotherm tended to become linear. The extent of adsorption, as measured by the distribution coefficient (Kd), increased with an increase in soil organic carbon. The rate of desorption of 2,4-D followed first-order reaction kinetics with respect to surface concentration, and decreased with an increase in the organic carbon content of the soils. The rate of degradation of 2,4-D, as measured by the half-life (t1/2), decreased with an initial increase in soil organic carbon, which is attributed to the increase in adsorption. With increasing adsorption, the rate of desorption decreased, resulting in a low concentration of 2,4-D in the soil solution that is available for microbial degradation. When the organic carbon content was more than 12%, however, both the adsorption and rate of degradation increased. The enhanced degradation of 2,4-D at these levels of organic carbon may be related to the increased biological activity of the soil, as measured by substrate-induced respiration, and the decreased 2,4-D-induced inhibitory effect on microbial activity.