Fixation of boric acid as a low toxicity wood preservative to decrease markedly its leachability was undertaken. Preservative systems based on the cross-linking and hardening of condensed tannins by hexamine, where boric acid is complexed onto the network, were found to markedly slow down the leaching of boron. They significantly enhanced wood durability before and after leaching once tested towards basidiomycetes according to the guidelines of EN 113. A particularly virulent fungal species was used for the tests. Further immersion of the wood so treated in hot unsaturated vegetable oil does yield comparable results.