Hydrothermal treatment of wood in a special water basin is employed before its mechanical processing for veneer production. Due to the wood autohydrolysis, the wastewater of the hydrothermal treatment is dramatically polluted, mainly with low-molecular lignin fragments, hemicelluloses and extractives. The waste biomass from the plywood basins may serve as a raw material for the development of new products with a value-added benefit. In this work, a chemical structure of lignin isolated from laboratory birch wood hydrolysate, imitating wastewater of plywood production, as well as its compositional features in comparison with commercial alkaline lignin has been studied.