Pine and beech samples were modified using various silicon chemicals. To find out fungicidal resistance the samples were exposed to 3 wood fungi: Coniophora puteana, Poria placenta and Trametes versicolor. Resistance to soft rot, bacteria and dynamic module of elasticity were studied also. After the modification and 8 weeks long exposition to soil beds, the smallest loss of elasticity module was found at pine samples, modified with MTES (100 %); and with TEOS (100 %) at beech. To find out whether the fixation time has any influence on fungi, EN 84 washing out test with water glass was performed. Microscopic research showed typical signs of either soft, brown or white rot. Modification means SMK2100-100 % and HS2090-20 %) were observed in the cells. Mini-block test with Coniophora puteana and Poria placenta did not succeed. The test with Trametes versicolor proved PTEO to be the most resistant modification means, the mass loss of the samples was less than 10 %.