Of two types of willow chip (28 and 50 mm cutting length), two types of willow chunk (100 and 200 mm cutting length) and one type of forest chip (28 mm cutting length), the pressure resistance to air flow during ventilation was measured. The influence of air velocity and particle size has been determined and the results are described by a mathematical equation. The measured pressure resistance correlated very well with the size distribution of the different chip and chunk types. The lowest pressure resistance was found in 200 mm long willow chunk at only 2 Pa/m at an air flow rate of 0.1 m/s, while the highest was found in the 28 mm chip of willow and forest material at 39 Pa/m. The pressure resistance was shown to be a factor 10 to 100 lower for these wood fuels than for agricultural crops like rare seed and grain. (C) 2000 Elsevier Science Ltd. All rights reserved.