Wood fuel powder cut in various types of impact mills and knife mills was characterised using sieve analysis, laser diffraction, image analysis and a funnel method for measuring the tendency to bridge. In addition, this study aims to find the relationship between particle characteristics, tendency to bridge and the type of mill. The different types of mills produced wood powder with different properties. Both sieve analysis and laser diffraction showed that impact mills produced wood powder with more fine particles than wood powder produced by knife mills. Image analysis showed that particle sizes and shapes differed between the two types of mills. The bridging tendency, measured as the size of a funnel opening needed for the powder to pass freely, was higher for powder produced by impact mills than by knife mills. The bridging tendency was found to be a combined effect of particle size and shape. (C) 2002 Elsevier Science B.V. All rights reserved.