Using isolated organosolv lignins from hardwood poplar and softwood lodgepole pine with varied physicochemical properties (molecular weight, aliphatic hydroxyl, phenolic hydroxyl, and hydrophobicity), the inhibitory effect of the lignins on enzymatic hydrolysis of cellulose was investigated and the relationship between lignin properties and the inhibitory effect was elucidated. The results indicated that the lignin inhibition to enzymatic hydrolysis of cellulose was closely related to the hydrophobicity and the phenolic hydroxyl groups of the lignin. The overall hydrophobicity of the lignin quantified by contact angle could serve as a predictor of the inhibitory effect of lignin. Hydrophilic modification of the lignin by carboxylation and sulfonation reduced the hydrophobicity by 22-30% and thereby removed the lignin inhibition by 76-96%. Phenolic hydroxyl group was a crucial factor affecting the inhibitory effect of lignin. Blocking free phenolic hydroxyl group by chemical reaction such as hydroxypropylation significantly (65-91%) reduced the inhibitory effect of lignin. (C) 2015 Wiley Periodicals, Inc.