The route of degradation of chlorthal-dimethyl was investigated using compound uniformly radiolabelled in the phenyl ring. In three aerobic soils (sandy loam to silty clay loam, pH 5.8-8.0, 0.8- 0.9% organic carbon), degradation was shown to proceed via MPA to TPA. Formation of bound residues was very small. Carbon dioxide was not trapped but material balances showed that there were no significant losses of radioactivity to the air. Studies in sterile soils demonstrated that degradation of chlorthal-dimethyl was biologically rather than chemically mediated. TPA is a major metabolite following aerobic degradation of chlorthal-dimethyl in soil with levels reaching a maximum level of 94.4% at study termination (197 days). A further study undertaken on the TPA metabolite in three soils showed that further degradation did occur. No further metabolites were identifiable and carbon dioxide and bound residues were formed in broadly equivalent amounts. MPA is also a major metabolite in soil. An average concentration of 11.3% MPA (n=2) was reached in TC sand loam soil at day 14. Also, in the first of the two TC sand loam soil studies, MPA exceeded 5% of the applied amount of active substance in two sequential measurements (reaching 8.9% and 5.8% at days 33 and 61, respectively). In a second study using TC sand loam soil under questionable sterile conditions, MPA reached 11.1 and 10.5% at days 95 and 137, respectively. In neither of the other soils examined (each tested in two different experiments) did MPA exceed 2% at any time. It is noted that TC sand loam soil type is considered as the most consistent to the SETAC (1995) recommendations for soil metabolism studies, in terms of pH, organic matter and clay content values, of all 3 soils used. Despite the fact that TPA and MPA have been found to be of no environmental concern (see section Ecotoxicology), they are toxicologically relevant and should therefore be included in the definition of residue relevant to the environment (soil). Under anaerobic conditions the route of degradation of chlorthal-dimethyl was the same as under aerobic conditions. No photolysis of chlorthal-dimethyl was observed on soil under conditions representative of natural sunlight.