The decay and termite resistance of boric acid (BA)- and di-sodium octoborate tetrahydrate (DOT)-treated sugi sapwood was tested in the context of additional heat treatments at two temperature levels. Heat treatments had no effect on boron release and almost all boron was leached from specimens during a 10-day weathering period. Decay tests with the brown-rot fungus Fomitopsis palustris and the white-rot fungus Trametes versicolor and a 3-week termite resistance test with the subterranean termite Coptotermes formosanus were performed. Heat treatments did not increase the decay resistance of either BA- or DOT-treated specimens against the brown-rot fungus. However, the decay resistance of BA-treated specimens against the same fungus increased after heat treatment at 220 degrees C for 2 h. Heat treatments at 180 degrees C for 4 h and 220 degrees C for 2 h also resulted in increased decay resistance of DOT-treated specimens against T. versicolor. Increased resistance against termite attack was observed only in DOT-treated specimens heated at 180 degrees C for 4 h or at 220 degrees C for 2 h. Accordingly, a synergistic effect between heat and DOT treatments was observed for resistance against white- rot decay and termites.