The aerobic route of degradation of dimethoate was studied in single radiolabelled study on one soil. The study was conducted according to the principles of GLP even though it was reported in 1989. The study was conducted at 22°C in the dark. During the course of the study, degradation of dimethoate was relatively rapid, reducing from 96.2% AR at day 0 to 1.9% AR at day 7. Low levels of metabolites were found, with two individual metabolites being identified. These were O-desmethyl dimethoate being found at maximum 2.0% AR at day 2 and O,O-dimethyl thiophosphoric acid found at maximum 0.7% also on day 2. ‘Other’ metabolites were detected at a maximum of 8.2% AR on day 4, but this fraction was composed of up to 13 components, with the maximum individual component at any time being 2.0% AR. It should be noted that the study used a single radiolabelling position, in the methoxy positions. The production of O,O-dimethyl thiophosphoric acid in this study indicates that a significant portion of the molecule was lost. This fragment is most likely to be N-methyl-2-sulfanyl acetamide. The notifier has argued that the carboxy- labelling position was used in the lysimeter and dark sediment/water studies and would have allowed detection of this metabolite if it was formed. This substance was used as a reference standard in the analysis of both studies and never detected in soil, leachate, water or sediment samples. Given the structure of this compound, it is likely that this would be subject to relatively rapid degradation and it would not be expected to accumulate to levels of any significance. Therefore the Rapporteur considers that sufficient information is available on the radiolabelled route of degradation of the active substance. Unextracted residues reached a maximum of 20.9% at day 30, with a trend for reduction thereafter, such that there was 16.2% AR unextracted at day 181; at day 90, unextracted residues were 16.0% AR. Total volatiles reached 74.7% AR at day 181, the majority (74.43% AR) being CO2; at day 90, volatiles were 70.2%, the majority of which would be CO2. Thus it appears that dimethoate is significantly mineralised under aerobic conditions. The mass balance was variable throughout the study, at times falling below 90% AR, but is generally considered acceptable as there was no consistent trend towards increasing loss as the study progressed.