The heterogeneity of lignin, especially the wide distribution of molecular weight, restricts the high value-added application of lignin. This work provided a convenient and efficient method to fractionate lignin by gradient acid precipitation. To investigate the acid precipitation performance of lignin with different molecular weight, four alkali-extracted lignin fractions from steam-exploded stalk were obtained from ultrafiltration and subsequently precipitated by acid addition. Results showed that molecular weight had a significant effect on the acid precipitation behaviors of lignin. High molecular weight lignin fractions (20 kDa retentate) were precipitated completely at higher pH and had narrow precipitation pH range compared with low molecular weight lignin fractions. The determination of Zeta potential and the particle size of lignin colloid revealed that higher Zeta potential and larger particle size formed by high molecular weight lignin accelerated the precipitation. According to the acid precipitation behaviors of different molecular weight lignin, gradient acid precipitation of lignin at pH 5.3, 4.0 and 2.0 was established and four lignin fractions were obtained. FT-IR and UV analysis showed similar chemical structures among the four fractions and increasing content of acidic groups as the molecular weight decreased. The GPC results indicated that the lignin fractions obtained from gradient acid precipitation showed gradually decreasing molecular weight from (M) over bar (w) 15,099-3053 g/mol and significantly decreasing polydispersity. Consequently, the study indicated gradient acid precipitation a convenient and successful technique to fractionate alkali-extracted lignin from steam-exploded stalk. (C) 2012 Elsevier B.V. All rights reserved.