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The oxidation kinetics of lignosulfonates (LS) from acidic magnesium-based sulfite pulping of Eucalyptus globuluswood with oxygen under alkaline conditions was studied. The analysis of oxidation products in the reaction system O2/NaOH revealed a predominance of aromatic aldehydes (vanillin and syringic aldehyde) though small amounts of vanillic and syringic acids and acetophenone/phenylacetaldehyde derivatives have also been detected. The rate constant of syringic aldehyde formation was roughly twice of that for vanillin. The effective activation energies for the oxidation of LS to aromatic aldehydes (ca. 60−70 kJ/mol) were rather different to those found for the formation of aromatic acids (ca. 110 kJ/mol) indicating different mechanisms involved in the rate-determining reaction step. The addition of catalyst (copper salt, 20% w/w) promoted the LS oxidation with increments of aromatic aldehyde yields by 25−50%. The maximum yields of syringic aldehyde and vanillin upon LS oxidation were 16.1 and 4.5%, respectively (150 °C, 20 min, PO2= 10 bar, 0.9 M NaOH solution). The highly negative effect of concomitant sugars in sulfite liquor to the yield of aromatic aldehydes was highlighted. [ABSTRACT FROM AUTHOR]