The lignin solubility in aqueous solutions of different ethanol concentrations was studied. The concept of the solubility parameter was applied to account for the effect of ethanol concentration on the lignin solubility. Experimental results showed that the lignin solubility increased strongly as the ethanol concentration increased from 9.5 to 47.5%; then, it increased much more slowly until a maximum was reached at an ethanol concentration of about 70%. Further increase in the ethanol concentration resulted in a slight decrease in the lignin solubility. Based on the lignin molecular formula, the solubility parameter (delta-value) of the ALCELL(R) lignin was 13.7 (cal/cm(3))(1/2). The delta-value of aqueous ethanol solutions of increased ethanol concentration was calculated and was found to decrease continuously from 22.31 (cal/cm(3))(1/2) for pure water to 12.08 (cal/cm(3))(1/2) for pure ethanol. The effect of ethanol concentration on the solubility of the ALCELL lignin was then explained based on the theory that lignin exhibited the maximum solubility when the delta-value of the solvent was close to that of the ALCELL lignin and the H-bonding capacity of the solutions with different ethanol concentrations was similar.