Summary:The effects of lenacil, terbacil, chlorthiamid and 2,4,5‐T at 100 ppm on carbon dioxide evolution, oxygen uptake and nitrogen transformation in two soils have been investigated for several months in the laboratory. The herbicides had no effect on CO2output from either Boddington Barn soil (organic carbon content 1.6%, pH 6.1) or Triangle soil (organic carbon content 3.7%, pH 4.8) apart from 2,4,5‐T which reduced it sometimes. All the herbicides caused temporary reductions in O2. uptake, but in Triangle soil treated with 2,4,5‐T a significant reduction was observed during the second half of the incubation. 2,4,5‐T and to a lesser extent chlorthiamid, reduced nitrification in Triangle soil. All the herbicides slightly increased mineralization of nitrogen except 2,4,5‐T which had variable effects in Tria