Solid biofuels such as wood chips are common fuels in many heating plants. The fuels are by nature heterogeneous and the fuel particles can form a stable structure across openings, which may stop the fuel flow in the feeding systems of heating plants. This phenomenon is known as bridging and in the present paper the bridging properties of several common wood fuels are studied and the influence of particle size and proportion of long particles, bed depth, raw material, species and moisture content is described. The fuels tendency to bridge was defined as the maximum slot opening over which a fuel will create a bridge. Measurements were done for wood fuels made from whole trees, logging residues and roundwood from coniferous species (Norway spruce, Sitka spruce and Contorta pine) and various deciduous trees and cut by two chippers to four different nominal particle sizes (30 and 35 mm chip and 50 and 100 mm chunk). The most important variable was the proportion of particles longer than 100 mm, which was a result of chipper type, knife setting and raw material. For most fuels, a higher moisture content increased the tendency to bridge. Wood fuels made from roundwood showed no significant correlation between tendency to bridge and moisture content. The study shows that the bridging problems can be reduced by adaptation of the fuel preparation to the given opening. (C) 2003 Elsevier Ltd. All rights reserved.