A high degree of water repellency is obtained in wood modified by silanation. However, the degrading effect of HC1, formed as a by-product in the reaction, may not be fully offset by the use of acid acceptors or inert gases in the reaction. Tensile tests on microtome sections of Pinus nigra wood, treated variously with dimethyldichlorosilane, clearly show that chemical modification substantially decreases tensile properties of wood. Experiments with large specimens of the same species suggest that silanated zone is only superficial. As a result, most of the strength properties of the large specimens remain unimpaired.