Isoxaben is degraded in soil mainly by microbial processes since no degradation of isoxaben was observed under sterile (dark) conditions. Photochemical transformation may also contribute to the degradation of isoxaben but this route is expected to be of minor importance. In laboratory studies, isoxaben was degraded substantially more rapidly under anaerobic conditions compared to aerobic conditions. Different metabolic pathways are also suggested for the aerobic and anaerobic processes. Four different soils were included in the aerobic study. Degradation of isoxaben proceeded via hydroxylation or oxidation of the methylpropyl side chain to give the hydroxy, hydroxyethyl or oxypropyl isoxaben metabolites, and demethylation of the aromatic methoxy groups to give the desmethyl isoxaben metabolite. The metabolite hydroxy isoxaben was identified at maximum levels ranging from 1.7 to 10.8% of the applied radioactivity and the amounts were increasing in all soils during the course of the study. Oxypropyl isoxaben was found as max. 1.3-7.3% and in one of the soils this metabolite was measured as > 5% of the applied radioactivity at two sequential measurements. In three other soils tested oxypropyl isoxaben generally showed a slowly increasing trend over the experimental period with the highest amounts measured towards the end of the study. Desmethyl isoxaben was only found as max. 1.6% and at a more or less constant level over the last sampling points. The fourth metabolite identified, hydroxyethyl isoxaben, was not identified in all soils and only at low amounts (max. 0.4% of the applied radioactivity). In the field studies hydroxy isoxaben and oxypropyl isoxaben were identified at similar levels. Based on these results, the aerobic metabolites hydroxy isoxaben and oxypropyl isoxaben are both considered to require further assessment. Under aerobic conditions, the non-extractable residues slowly increased during the course of the study, to max. 7.2-15.8% of the applied radioactivity (day 122) when U-phenyl-14C radiolabel was used, and max. 20.4% (day 122) when isoxazole-14C radiolabel was used. These non-extracted residues consist of humin bound radioactivity and small amounts of isoxaben and its oxypropyl and hydroxy metabolites which were associated with the fulvic acid fraction. Mineralisation reached 1.6-3-4% (U-phenyl-14C radiolabel) and 8.6% (isoxazole-14C radiolabel) after 122 days. Apart from CO2, the production of volatiles was negligible.