Zinc oxide as well as lead (II), lead (IV) and tin (II) oxide have been found to react with guaiacol as a simple lignin model compound to form complexes. In comparison with results obtained with chromium trioxide, only zinc oxide is able to give worthwhile results when used to waterproof wood. Because of the “dimeric” character of the complex formed with lignin, zinc oxide is a less efficient wood waterproofer than CrO3, which instead forms “polymeric” complexes. A theory of the preservation effect of wood by metallic ion is advanced. The results obtained indicate that metallic compounds which are capable of further cross-linking the lignin network by forming insoluble complexes with it can act as wood waterproofers. Metallic compounds giving insoluble monomeric complexes not enhancing lignin cross-linking or not giving insoluble complexes do not act as wood waterproofer.