Three European soils and one US soil were incubated with radiolabelled BYH 18636 for 365 d under aerobic conditions in the dark at 20 °C (all soils) and a soil moisture of 43–50 % of maximum water holding capacity (EU soils) and 79 % of 1/3 bar water holding capacity (US soil). The applied radioactivity (AR) was completely recovered (92.7–102.4 %). After 365 d, non-extractable radioactivity ranged from 38.8–56.9 % AR for label A (dihydrotriazole-3-14C) and from 25.8–26.8 % AR for label B (thiophene-4-14C). Non-extractable residues were of heterogeneous nature, associated with humic and fulvic acids, and with insoluble humin. Both radiolabelled moieties of the test compound were significantly mineralized. After 365 d, 14CO2 accounted for 11.6–38.9 % AR for label A, and for 25.0– 59.4 % AR for label B. No significant amounts of organic volatile degradates were found (≤ 0.1 % AR). Major soil metabolites were BYH 18636-carboxylic acid (maximum 52.3–53.6 % AR), BYH 18636-MMT (maximum 20.6 % AR), BYH 18636-sulfonamide (maximum 15.2 % AR) and BYH 18636-sulfonamide-carboxylic acid (maximum 21.2 % AR). BYH 18636-triazolinone-carboxamide and BYH 18636-thienosaccharine were only identified at exaggerated rates. There was only little unidentified radioactivity. The largest individual unidentified component was 3.4 % AR polar radioactivity. Breakdown of BYH 18636 in soil involves two major routes (1 and 2) and one minor route (3) with the following initial steps: (1) Separation of the dihydrotriazole- and thiophene moieties by cleavage of the carboxamide bond. (2) Initial cleavage of the thiophene ring methyl ester function. (3) A minor degradation pathway may involve an alternative separation of the heterocycle moieties of BYH 18636 and/or BYH 18636-carboxylic acid, by cleavage of the connecting sulfonamide bond (instead of the carboxamide bond as in pathways 1 and 2).