The route of aerobic degradation of propisochlor has been examined in an acceptable study on four soils (sandy loam to clay, pH 6.2 - 7.34, 1.02 -1.89 %0C) incubated at 20°C and 40% MWHC. Two different radiolabel positions were investigated (phenyl ring and chloroacetyl) and, although there were differences in the amount of C02 (5.6-17% after 120 days ring labelled, 18-39% after 120 days chloroacetyl labelled) and bound residues (34-41% after 120 days ring labelled, 23-27% after 120 days chloroacetyl labelled) formed with each label, the identified metabolites were the same for both labels. This confirms that no cleavage of the chloroacetanilide structure occurs. Degradation proceeds via oxidation of the chloro group to the metabolite N-(2-ethyl-6-methylphenyl)-2-hydroxy-N-(isopropoxymethyl) acetamide (MS), followed by further oxidation to form [(2-ethyl-6-methylphenyl) (isopropoxymethyl)amino](oxo)acetic acid (M4). Concurrently, propisochlor degrades (via conjugation with glutathione) to ({2-[(2-ethyl-6-methylphenyl) (isopropoxy-methyl)amino]-2-oxoethyl}sulfinyl)acetic acid (M7), N-(2-ethyl-6- methylphenyl)-N-(isopropoxy-methyl)-2-(methylsulfonyl)acetamide (M6) and 2-[(2-ethyl-6-methyl phenyl)(isopropoxymethyl)amino]-2-oxoethane sulfonic acid (M2), which is further degraded to 2-[(2-ethyl-6-methylphenyl)amino]-2-oxoethane sulfonic acid (M1) by cleavage of the isopropoxymethyl side chain.