AbstractThe transformation of aldicarb sulfoxide and aldicarb sulfone was studied in incubations with water‐saturated subsoils under simulated field conditions at 10°C. The subsoils were collected at four locations from beneath the water table at a depth of 2.5 to 3.5 m. In three of the subsoils, the half‐life of sulfoxide, incubated at concentrations of 0.14‐0.17 mg litre−1, ranged from 0.7 to 2.8 years. At higher concentrations (8‐13 mg litre−1), its half‐life ranged from 3.4 to 6.4 years. At the lower concentration, a large fraction of sulfoxide was transformed into sulfone. The rates of transformation of the sulfone at the lower concentration in the three subsoils corresponded to half‐lives of 3.3 to 8.1 years, but in only one subsoil was a significant transformation rate (half‐life 6.7 years) measured at the higher concentration during the 2.3‐year incubation period. The half‐lives at the lower concentrations were more like those in field studies, and perhaps would still underestimate transformation rates under field conditions. After a year, 2.5‐15% of the higher sulfoxide and sulfone doses had been trapped as [14C] carbon dioxide. In the fourth subsoil, with more anaerobic conditions, the half‐life of sulfoxide at both concentrations was less than 0.02 year and that of sulfone was about 0.04 year. Four or five radio‐labelled transformation products could be traced in this subsoil and about half of the dose of both compounds was trapped as [14C] carbon dioxide.