SummaryA newly developed chemical assay was used to determine the degradation in soil of two sulfonylurea herbicides, primisulfuron‐methyl and metsulfuron‐methyl, under both controlled and field conditions. The results from the chemical assay were compared with those from traditional bio‐assays for determination of persistence in the field. Phytotoxic effects of these herbicides were not observed after 6 weeks following application to an acidic (pH 5.7) soil with high organic matter content (7.3% o.c). Half‐lives of 13 to 29 days were measured for primisulfuron‐methyl at different soil‐water contents and temperatures while those for metsulfuron‐methyl ranged from 8 to 36 days. The rate of degradation of metsulfuron‐methyl was more sensitive to temperature than that of primisulfuron‐methyl. Persistence in the field was shorter than expected considering the results from the controlled environment studies. However, determination of the persistence by both chemical assay and bioassay methods produced ver