Calorimetry was applied to develop a comparative study between the basal respiration and the degradation of an external carbon source in several soil samples. The main goals were to search for the connection between these reactions in soils and to test the sensitivity of calorimetric indices for the microbial activity. The soil samples were from the Brazilian Amazon and from Galicia (Northwest Spain). The reactions monitored were the aerobic degradation of the organic matter and the aerobic degradation of glucose. The results showed that both reactions are closely correlated when they were studied by specific indices related to the biomass, and that the calorimetric indices calculated were sensitive to different intrinsic soil properties. (C) 2007 Elsevier B.V. All rights reserved.