The pesticide 2,4-D (2,4-dichlolorophenoxyacetic acid) was immobilized on a silica gel surface to yield a new compound designed SiD. The free and anchored pesticide was applied to typical Brazilian agricultural soils and the biodegradation caused by microbial activity was followed by microcalorimetry. The activity of the microorganisms on soil samples under 34.8% of controlled humidity at 298.15 +/- 0.02 K was determined through power-time curves. The recorded curves for increasing amounts of the 2,4-D active principle varied from zero to 6.67 mg per gram of soil. The increased amounts of both free and immobilized pesticide caused an enhancement of the original thermal effect. The calorimetric data demonstrated that the anchored pesticide (SiD) presents a much lower biodegradation with microbial activity, when compared with the free pesticide 2,4-D).