Microcalorimetry was applied to the study of the microbial activity of four soils with different percentages of organic matter. The qualitative study of the heat flow rate-time curves, recorded from soil samples amended with glucose, showed remarkable differences in the soil microbial activity. In order to show results in a more quantitative way, the total heat evolution, Q(tot), the total heat evolution of soil samples amended with glucose, Q(tot(glc)), and the values of peak time and microbial growth rate constant, mu, were calculated from the heat flow rate-time curves recorded from all samples. Values of peak time increased with decreased microbial density and a positive correlation between total heat evolution, Q(tot), and percentage of organic matter, was found. Microcalorimetry appears as a suitable technique to carry out both qualitative and quantitative comparative studies of microbial activity in soils.