AbstractImazosulfuron, 1‐(2‐chloroimidazo[1,2‐a]pyridin‐3‐ylsulfonyl)‐3‐(4,6‐dimethoxypyrimidin‐2‐yl)urea, is a new post‐emergence sulfonylurea herbicide applied once per growing season, highly active at low application levels, used to control most annual and perennial broad‐leaf weeds and sedges in paddy rice (75–95 g AI ha−1) and turf (500–1000 g AI ha−1), and characterised by low mammalian toxicity. The degradation of imazosulfuron in a sandy loam soil (10–15 cm depth) has been investigated under aerobic and anaerobic laboratory conditions to evaluate its likely environmental fate. Test soils were treated with imazosulfuron in acetonitrile to give a final concentration of 0.2 mg kg−1 (100 g ha−1). The treated soils were extracted with methylene chloride + acetonitrile + 1 M ammonium hydroxide (4 + 15 + 1 by volume) and analysed by HPLC. Recoveries from spiked soils ranged from 80% to 95% (RSD 0.5–2.5%; n = 3) for imazosulfuron and its metabolites 2‐chloroimidazo[1,2‐a]pyridin‐3‐sulfonamide (IPSN), 1‐(2‐chloroimidazo[1,2‐a]pyridin‐3‐ylsulfonyl)‐3‐(4‐hydroxy‐6‐methoxypyrimidin‐2‐yl)urea (HMS) and 2‐amino‐4,6‐dimethoxypyrimidine (ADPM). The limits of quantitation ranged from 0.001 mg kg−1 to 0.005 mg kg−1. Chemical cleavage of the sulfonylurea bond to give ADPM and IPSN was the main degradation pathway in aerobic conditions, whereas a remarkable production of HMS under anaerobic conditions suggested that degradation was due to micro‐organisms which had the ability to demethylate imazosulfuron. In aerobic and anaerobic conditions, imazosulfuron disappeared from the soil with a half‐life of approximately 70 (90% CL 62–75; r2 = 0.98) and 4 (90% CL 3.2–4.2; r2 = 0.97) days, respectively.© 2001 Society of Chemical Industry