Biochar, a solid product of biomass pyrolysis, is a promising concept for climate change mitigation and adaptation, as it can sequester atmospheric CO2 while improving quality of soil where it is stored. However, for this potential to be realised, it is necessary for biochar to have high environmental stability, i.e., resist various decomposition processes over long time. The main objective of this work has been to relate biochar production conditions to the yield, and properties of biochar, particularly its long-term stability. We used our lab-scale pyrolysis facilities to produce biochar at three temperatures between 350 degrees C and 550 degrees C, from selected feedstock (pine, mixed larch and spruce chips, softwood pellets). We measured the yield of biochar and then used an accelerated ageing assay to obtain information on the stability of biochar. Such information is very important for the assessment of the climate change mitigation potential of biochar, as it has not yet been clearly defined what proportion of biochar actually remains "permanently" sequestered and how much is released back to the atmosphere in the short to medium term. The results of this work showed that despite increase in the stability of biochar with increasing pyrolysis temperature, the yield of stable biochar fraction is nearly independent of the temperature. These findings are essential for the optimisation of pyrolysis conditions for production of biochar with selected properties, as well as for modelling biochar systems and their climate change mitigation potential as compared to other uses of biomass, such as bioenergy, biofuels and/or chemicals. (C) 2011 Elsevier Ltd. All rights reserved.