The goal of this study was to investigate the influence on physical and mechanical properties and on biological durability of beech wood (Fagus sylvatica) modified with polyethylene glycol (PEG) 400 as an alcohol and various carboxylic acids (citric acid, malic acid, 1,2,3,4-butanetetracarboxylic acid). PEG 400, chosen for its large and flexible structure, was expected to minimize negative effects on mechanical properties. However, contrary to expectations, the tensile and impact bending strength decreased – similar to other modifications. The tensile strength decreased (36.0% to 46.3%), while the modulus of rupture did not show a strong decrease (2.4% to 15.2%). The compression strength (11.7% to 40.6%) and hardness (6.0% to 17.1%) increased. The impact bending strength reduced strongly (43.0% to 69.3%). The modifying agents, fixed inside the wood, demonstrated a high anti-swelling efficiency during leaching cycles (27.1% to 45.9%). Decay resistance against brown, white and soft rot fungi was improved. Modification with PEG and citric acid falls into DC 1-2, modification with PEG and malic acid into DC 2-4 and modification with PEG and 1,2,3,4-butanetetracarboxylic acid in DC 1 for brown rot. In case of white rot, all modifications can be classified as DC 1.