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The effect of thermo-hydro treatment (THT) on the durability of birch wood veneers and two plywood products - A and B was studied. THT was carried out in a multi-functional pilot scale wood modification device under elevated water vapour pressure at the following temperatures and treatment times (°C/min): 150/10; 150/50; 160/10 and 160/50. After THT, durability was examined according to the standards ENV 12038, EN 84 and EN 73. The durability class (DC) according to the CEN/TS 15083-1 was assigned to each treatment after fungal exposure. Water soluble extractives from THT samples were analysed with a UV-Vis Spectrometer in a range of 200 - 400nm, potentiometric titration and the Malaprade reaction. The best DC (2 - 3) for aged veneers was reached with the THT 160°C against the white rot fungus. The THT of industrial plywood A at 160°C enabled producing a material with improved durability (DC 1 - 3) after ageing. Plywood B with glued THT veneers showed lower decay resistance than plywood A. After ageing, better performance of plywood B was achieved at the THT 160°C assigning to DC 2 - 4. At the THT regimes under study, water soluble products (acids, sugars, aromatic compounds) were formed from both polysaccharides and lignin. [ABSTRACT FROM AUTHOR]