Nicosulfuron was moderately persistent in soils when treated at the proposed field rate (60 g/ha); there was no obvious correlation between DT50 and soil pH in these particular studies, although the studies may be criticised as a limited pH range was used (pH range 5.3 – 6.6). Aqueous hydrolysis studies show that nicosulfuron degrades much quicker in acidic conditions compared with neutral or alkali conditions. Degradation produced five polar metabolites, the maximum amounts seen in the route of degradation studies were: 14.4 % AR HMUD, 7.2 % AR ADMP, 21.5 % AR ASDM, 26.8 % AR AUSN and 11 % AR UCSN. The maximum accumulation of unextracted residues at 112 days was 45.9 % AR with little difference between radio labelling position. Mineralisation to CO2 (16.8 % AR at 112 days) was evident in the studies with [14C pyrimidine] nicosulfuron, considerably less [14C]CO2 was evolved in studies with the [14C pyridine] labelled study (1.3 % AR at 112 days). The sequence of metabolite accumulation is consistent with an initial demethylation of a pyrimidinyl methyoxy group to form HMUD (maximum 14 %) followed by cleavage of the pyrimidinyl ring to produce AUSN (maximum 27 %) and UCSN (maximum 11 %). These latter two compounds were characterised in this study as major metabolites in the pyridine labelled study and may have accounted for a part of the uncharacterised polar metabolites in this study. Additionally, ASDM (maximum 22 %) produced by cleavage of the sulfonylurea bridge, was identified in the pyridine labelled study as a major metabolite. ADMP (maximum 7.2 %), which is also produced by cleavage of the sulfonylurea bridge was found in studies conducted with [14C pyrimidine] labelled nicosulfuron, although this never accumulated to >5 % on two consecutive time points in the route of degradation study. In the rate of degradation studies (Mamouni & Galicia, 1991) ADMP accumulated to a maximum of 26.9 % in one study and therefore must be regarded as a major metabolite.