The modified chemical, physical and structural properties of wood after heat treatment can affect the bonding process with adhesives. The objective of this research was to determine to what extent the degree of heat treatment influenced the bonding of treated wood with phenol-formaldehyde (PF) adhesive. Spruce (Picea abies Karst) lamellas were heat treated at 180 degrees C and 220 degrees C and then bonded. The shear strength and wood failure of the differently pretreated specimens were determined. Wettability and penetration of the adhesive were also investigated. The results showed that the shear strength of the PF adhesive bond was influenced by the heat treatment of the spruce wood, and depended on the type of pretreatment of the wood specimen prior to testing. The observed reduction in the shear strength of the PF adhesive bond was ascribed to a decrease in the wood strength itself, caused by the heat treatment, and also to other effects induced by exposure of the wood to elevated temperatures. The changes in wettability and adhesive penetration did not significantly influence the bonding process of the heat-treated wood with the PF adhesive. (C) Koninklijke Brill NV, Leiden, 2010