The phenomenon of off-gassing from wood pellets during storage has been the cause of several, in some cases fatal, accidents due to toxic atmospheres in storages. To optimize safety measures the nature of the responsible processes needs to be clarified. In this study the impact of O-2 availability, which is a decisive factor for the presumed oxidation of fatty acids, is pointed out. Off-gassing rates of CO, CO2, VOC, and CH4 of pellets at relatively constant O-2 levels of approximately 35%, 20%, and <1% over a period of 20 d at approximately 295 K were investigated. For this purpose 7 kg of spruce pellets was stored under simulated ventilation of the atmosphere in a 31 L tank. Gas concentrations were determined every 24 h by GC-FID/TCD. Compared to the mean emission rates at 35% O-2 of CO (0.22 mg kg(-1)pellets(d.b.) in 24 h) and CO2 (0.76 mg kg(-1)pellets(d.b.) in 24 h) the lowest 02 concentration of <1% resulted in a significant reduction of off-gassing rates of 40% for both gases. In contrast the release rates of VOCs and also CH4 decreased with the higher O-2 concentration (0.035 to 0.025 mg kg(-1)pellets(d.b.) in 24 h; 0.0085 to 0.0061 mg kg(-1)pellets(d.b.) in 24 h), presumably, because of increased onward reactions to CO and CO2. Since off gassing was not prevented by the lack of 02 (<1% O-2-trial) it is assumed that the O-2 required for the reactions originated from the biomass itself. During the storage of pellets at 20% O-2, emission rates of CO (0.18 mg kg(-1)pellets(d.b.) in 24 h) and CO2 (0.79 mg kg(-1)pellets(d.b.) in 24 h) at the start decreased by more than 20% and those for VOCs (0.032 mg kg(-1)pellets(d.b.) in 24 h) by almost 30% after 3 weeks. It can be assumed that in ventilated storages the reactivity and thus a potential risk from off-gases from wood pellets decreases considerably in only a few weeks. The effects of aging, in terms of declining reactivity at relatively constant tank conditions, on off-gassing rates could be clarified for the first time. A realistic development of the decline of reactivity of the material itself could be determined.