By varying the storage conditions for lab-scale samples of different biomasses, the mircrobial growth in the samples varied. The microbial acitvity caused heat release and organic emissions were sampled. By measuring the heat release with isothermal calorimetry and organic emissions with solid phase microextraction (SPME) and gas chromatorgraphy with mass spectrometry (GC-MS), it may be posssible to predict the deterioriation in fuel quality, substance losses and health effects for a special fuel storage. The biofuels investigated were pellets made from peat/wood, bark/wood and wood pellets.