Different sonication times (15, 30,60 and 120 min) were applied in aqueous media to olive tree pruning residues lignin obtained by alkaline fractionation (7.5% w/w NaOH, solid to liquid ratio 1:10 w/w, 90 min, 85 degrees C). The observed sample solubilisation ratio due to ultrasonic irradiation was 24-28%. The resulted liquid fractions were analyzed by HPLC in order to determine their mono- and polysaccharides content (up to 0.404 and 3.393 g/L, respectively). The insoluble solid fractions were chemically characterized, and their chemical structure (ATR-IR, GPC), thermal behavior (TGA) and antioxidant activity (Folin-Ciocalteu and ABTS assays) were studied. The results showed that ultrasounds cleaved lignin-carbohydrate linkages and promoted hemicellulosic impurities removal from the alkaline sample (up to 32% of initial carbohydrates content), but also affected to insoluble lignin content (19% of Klason lignin solubilisation). (C) 2012 Elsevier B.V. All rights reserved.