Studies were conducted to investigate the desorption of diuron and isoproturon adsorbed on undispersed clay loam soil, and the inÑuence of residence time in soil on desorption. The soil was treated at 0É6 or 3 mg kg~1, at 70% moisture content and in the presence of sodium azide to prevent degradation. Measurement of herbicide concentrations in soil solution sampled by means of glass microÐbre Ðlters showed that adsorption mainly occurred for one day but long-term sorption proceeded for [two weeks. After a one-day or threeweek residence time, soil solution was partly replaced (28%). Measurement of concentrations in solution showed rapid desorption, with equilibria being achieved within 1 h (diuron) or a few hours (isoproturon). After 16 successive desorptions done at 30-min or 12-h intervals, equilibration times tended to be longer. For the short residence time, desorption and long-term sorption could occur simultaneously and equilibration might be faster. Residence time had no signiÐcant e †ect on desorption kinetics nor on the small hysteresis observed for diuron. The aging e †ect, involving long-term sorption only, decreased the proportion of diuron removed from the soil by successive desorptions but, for isoproturon, desorption frequency and desorption kinetics were more important.