Abstract The effect of chemical modification with acetic or hexanoic anhydride upon the decay resistance of wood was studied. Both sapwoods and heartwoods of the following trees were investigated: Japanese larch, Larix kaempferi (Lamb.) Carrière; Korean pine, Pinus koraiensis Siebold et Zucc. as softwoods and European beech (only sapwood), Fagus sylvatica L.; oriental white oak, Quercus aliena (Blume) as hardwoods. After chemical modification, the samples were exposed to the brown rot fungus Coniophora puteana (FPRL 11E). The study investigated whether weight percentage gain or degree of hydroxyl substitution were the main factors controlling decay resistance. It was found that decay resistance is associated primarily with cell wall bulking rather than hydroxyl substitution. However, there are differences in behaviour between the acetylated and hexanoylated wood and the possible reasons for these differences are discussed.