The microstructure of pine wood at the first stage (10 days) of exposure to brown-rot (Gloeophyllum trabeum and Porta placenta) and white-rot (Coriolus versicolor) fungi after extraction of low-molecular hydrophilic products of degradation by hot water was investigated by the water vapour sorption method. The content and composition of monosaccharides in the extracts were determined by the GLC-method. After washing of sound wood, the pore volume of the sample in the region of pore sizes D ~ 1.5 nm increases. The same maximum appears on the curves of pore volume distribution in size after exposure of wood to brown-rot fungi: after washing, this maximum is shifted into the region of lesser pore sizes. The extraction of low-molecular products with water does not change essentially the wood structure, but results in the enhancement of its hydrophility. The most essential changes in the wood microstructure are caused by white rot, namely, the wood surface becomes more hydrophobic, and the pore volumes in the whole region of their sizes decrease dramatically. The composition of monosaccharides in the water extracts of the samples decayed by P. placenta and C. versicolor, as well as the control sample is the same (xylose, arabinose and galactose); only after the exposure to G. trabeum, new components (glucose and mannose) appear besides the above-mentioned ones.