Environmental pressures in France and in most European countries during the last decade have led to the development of more environmentally acceptable preservation methods. In this context, wood heat treatment is one of the most investigated alternative methods. Important chemical modifications resulting from thermo-degradation reactions confer new properties to wood, like increased decay resistance or higher dimensional stability, while mechanical properties like tensile strength decrease after treatment. However, the improved durability of heat treated wood is not sufficient to allow its utilization in ground contact, where it is subjected to insect and fungi attacks. Impregnation with borax before thermal treatment could be an interesting method to improve the properties of thermally modified wood. Boron is a relatively harmless biocide that improves resistance to fungi and insects like termites. Additionally, borax can also improve wood fire resistance due to its fire retardant effect. To reduce boron leachability, two additives previously developed in the laboratory corresponding to water soluble polymerizable polyglycerol derivatives were added to the borax solution, taking advantage of thermal treatment to fix the latter through polymerization within the wood structure limiting boron leachability.