The availability of pesticides in the soil is one of the factors that determine their mobilization and transport in runoff and groundwaters. In this paper, we aim to better understand the change in pesticide availability with increasing residence time in the soil. The relative contributions of degradation and sorption to the overall fate of molecules were examined, as was the remobilization of residues from the topsoil in water. The experiment was undertaken in undisturbed soil micro-lysimeters under natural climatic conditions for 1 year. We applied 14C-diuron to the soil surface to trace its residues in water-extractable (W), methanol-only-extractable (MO) and non-extractable (N) fractions and in runoff waters. Mass-balance calculations were fitted to the observed data to determine the quantities degraded or transferred within and between each of the compartments studied. Losses of diuron residues in runoff declined exponentially from 473 to 4 µg l−1 in 1 year. After 80 days, sorption of diuron residues in the soil was significant, as was degradation in the W and MO pools. Losses in runoff were small because of sparse rainfall. The following 140 days corresponded to a humid period with increased degradation in both W and MO pools. A redistribution from the MO towards the W pool occurred, probably because of intense rainfall at that time, leading to an apparent desorption of diuron residues. This was the most important result, suggesting that ageing processes should not be considered irreversible over time. The last phase of change, which occurred during winter, was slow. Leaching processes appeared to be of minor importance for diuron, with the surface layer containing more than 84% of diuron residues