Mancozeb is a fungicide frequently used in tropical countries. It rapidly decomposes into ethylenethiourea (ETU), a more stable and toxic metabolite than mancozeb that is, therefore, regarded as a pollutant of concern. The objective was to study ETU formation and decay kinetics in soil and water under tropical conditions in order to assess its potential for accumulation. Batch experiments, spiked with either mancozeb or ETU, were carried out under natural (= active) as well as tyndallized conditions. In active soils, dissipation of ETU occurred significantly faster (half-life 1.5 h) than in tyndallized soils (half-life time 28 h). In water under natural and sterile conditions, decay was slower than in soils with an ETU half-life time of 115 and 99 h, respectively. Microbial activity was seen to play an important role in ETU dissipation in soil. However, in water nonbiological processes seem to be more important in the breakdown of the molecule, with hydrolysis being the most probable decay mechanism. Decay of both mancozeb and ETU was found to occur more rapidly than previously reported. The high humidity and temperatures under the simulated humid tropical conditions, and higher microbial activity, lead to more rapid decay of these molecules than under other conditions. Nevertheless, a concentration of 1.29 mg ETU L–1 was still observed 8 d after adding mancozeb (20.83 mg L–1) to water under humid tropical conditions. These results suggest that, in comparable regions in the humid tropics, it is unlikely that ETU would accumulate in soil but it represents a potential risk for accumulation in water bodies.