The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was applied to a typical Brazilian red Latosol soil. The thermal effects evolved by 2,4-D degradation were followed in a series of microcalorimetric experiments. The power-time curves were recorded for increasing doses of herbicide. varying from 0 to 6.67 mg g(-1) under a 34.8% controlled humidity. The herbicide was used as a food source by soil microbes, the degradation causing the thermal effect. The thermal effect increases to 2.67 mg g(-1). Above this concentration, a decrease of the thermal effect was observed due to toxicity of 2,4-D to the microbes.