We have developed a method based on isothermal calorimetry to monitor the colonization of wood materials by decay fungi. The heat produced is a direct and continuous measure of the activity (respiration) of the fungus. This pre-study shows that wood materials treated in different ways give results that reflect the known ranking of the materials regarding resistance to decay. In addition to this, the 40 days measurements show several phases and periods of oscillating respiration. As the calorimeter continuously monitors the colonization and degradation process, this method provides more information than classical/standardized decay tests in which typically only the final mass loss is determined.