Isothermal microcalorimetry is now established as a useful technique for the characterization of the microbial activity in soil. A brief summary of publications from this field and of instruments used in such work is presented. Several experimental parameters that can form important sources for systematic errors are discussed and it is suggested that further method work is made in this area. In most isothermal microcalorimetric investigations on the microbial activity in soil, the samples are amended with glucose. It is proposed that cellulose also will be used.