A series of microcalorimetry experiments were performed to follow the effect of moisture on soil microbial activity. Activity was stimulated by adding a solution of 6.0 mg of glucose and 6.0 mg of ammonium sulfate in 0.80 cm(3) of distilled water to 1.50 g of red Latosol soil. This solution represents the moisture used in the experimental system (MAES) which directly affects the microbial activity. MAES values vary from 9.1 to 50.0% in selected reports where the humidity was not taken into account in calorimetric determinations. In this study, the thermal power caused by microbial activity was followed in samples of red Latosol with MAES values of 34.78%. In soil with absolute humidity values varying between 36.74 to 37.74%, the total heat reached a maximum value, 15.92 J g(-1). A drastic decrease above absolute moisture values of 41.82% was observed. The methodology with 34.78% MAES values showed a better microbial activity on soil than lower values of humidity. The proposed methodology establishes a defined MAES value of 34.78% to optimize the calorimetric results obtained from air-dried soil experiments.