Imazosulfuron, 1-(2-chloroimidazo[1,2-alpha]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-alpha]pyridin-3-sulfonamide (IPSN), 1-(2-chloroimidazo[1,2-alpha]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.