Self-ignition in storde wood based biofuels is an environmental as well as an economical problem. The amount of biofuels, which is going to be stored, might increase due to the decision taken by the European Union that the emissions of greenhouse gases should decrease. Storage of oragnic material leads to chemical as well as microbial activities. The heat released from aged sawdust (stored outside for three months), dried mixture before it was pressed to pellets and three different kinds of pellets due to microbial activity was measurede with an isothermal calorimeter (TAM Air) under various conditions.- Experiments with and without addition of Cu,Fe and Mn were performed in order to investigate if a catalytical effect could be observed. The results showed that the addition of copper had different efffects depending on which wood material that was studied and the temperature at which the samples had been stored. The addition of Fe or Mn generally did not give any measurable increase in the heat released.