AbstractThe loss of aldicarb sulphoxide was studied in incubation experiments with soil from four plough layers and two deeper layers. The loss during the 111 days of the experiment could be described by first‐order kinetics. The half‐lives at 15°C ranged from 20 days in a clay loam to 46 days in a peaty sand. The loss of sulphoxide in deeper layers was considerably slower than in the corresponding top layers of a soil profile. In soil from a silty layer at 70–90 cm depth the half‐life was about 53 days. In soil from a sand layer at 90–110 cm depth a loss of only about 15% was measured after 111 days of incubation. First‐order rate constants for sulphoxide conversion in a clay loam at 6, 15 and 25°C were found to be 0.009, 0.033, and 0.05 day−1 respectively; in a greenhouse soil these rate constants were 0.0052, 0.019 and 0.04 day−1 respectively. The fractions of aldicarb sulphoxide that were oxidised to sulphone at 15°C in soil from plough layers were computed to range from 0.52 to 0.76.