The route and rate of degradation of pyroxsulam was investigated in a series of radiolabelled and non-radiolabelled laboratory studies under aerobic, sterile and anaerobic conditions; the potential effect of sunlight on degradation was also studied. A standard aerobic degradation study under non-sterile laboratory conditions led to pyroxsulam degrading to <5% AR after 133 days. Aerobic degradation led to the formation of five metabolites: 7-OH-XDE-742 (max 13.7% AR, 3 DAT), 5-OH-XDE- 742 (max 24.1% AR, 3 DAT), 6-Cl-7-OH-XDE-742 (max 26.2% AR, 7 DAT) and the minor metabolite PSA (max 5.9% AR, 29 DAT). In this study non extractable residues (NERs) were formed at a high level (max 89.6% AR, 133 DAT); the two soils with the most alkali pH values (LUFA and Bruch West), had the highest levels of NERs with <5% mineralisation and >70% non-extracted residues at 100 days. The significant metabolites identified in the aerobic degradation studies were 7-OHXDE- 742, 5-OH-XDE-742, 6-Cl-7-OH-XDE-742, PSA and pyridine sulfonamide. In aerobic soil the 5-OH-XDE-742 metabolite reached a maximum of 24.4%AR, the 7- OH-XDE-742 metabolite reached a maximum of 13.7%AR, the 6-Cl-7-OH-XDE-742 reached a maximum of 26.2%AR. Pyridine-sulfonamide reached a maximum concentration of 13.2%AR. Pyridine cyanosulfonamide reached >5% on two separate time points in one soil, though these were not consecutive, (max. 81.%AR) and the sulfonic acid metabolite (PSA) was >5% at 2 consecutive time points in one soil (max. 5.9%). Mineralisation to CO2 was up to 15.6%AR and NER reached a maximum of 91.6%AR after 133 days.