Microcalorimetry has been used to study the effect of moisture on soil microbial activity. Different moisture regimes produce changes on heat flow rate-time curves recorded for soil samples, affecting also certain parameters, such as the total heat evolution Q(tot), the microbial growth rate constant mu and growth yield Y, which are calculated by the microcalorimetric method. A positive correlation was found between the percentage humidity, the total heat evolution and the microbial growth rate constant. The values of these parameters decrease with the dryness of the soil. Field capacity moisture appears to be the optimum moisture level for soil microbial activity.