Abstract Lignosulfonates (LS) from acidic magnesium-based sulfite cooking of Eucalyptus globulus wood were modified via laccase-catalyzed oxidative treatment with the aim to improve their performance as plasticizing additives in concrete formulations. The target parameters were the increment of molecular weight (M_w) and the amount of oxidized groups. The laccase-assisted oxidation under optimized conditions (40°C, pH 4.5, enzyme loads of 83–500 U g⁻¹ for 90 min) allowed M_w increment up to 11-fold and almost doubled the amount of carbonyl and carboxyl groups without using any mediators. Modified LS maintained their solubility in water and possessed a zeta- potential close to that of initial LS. The characterization of modified LS has been carried out by ultraviolet-visible (UV-Vis), Fourier transform infrared-attenuated total reflectance (FTIR-ATR) and quantitative ¹³C nuclear magnetic resonance (NMR) spectroscopy, and the sulfonic and phenolic groups were assessed by conductometric titration. It was concluded that LS polymerization occurred mostly via the formation of new aryl ether bonds (two thirds of the modification) and biphenyl bonds (the remaining third). However, part of the newly formed bonds of unknown origin are temperature labile and cleaved during the concentration of LS at pH 4 and 80°C under vacuum, which led to the reduction of the M_w of the modified lignin to almost one third.