The effect of lignin and related compounds on the hydrolysis of the cellulose model compound (carboxymethylcellulose) by cellulase was investigated. Lignin, black liquor from the kraft pulping process and guaiacol, vanillin, vanillic acid, acetovanillone and protocatechuic acid were studied. Although the Lignin related compounds affect differently cellulose hydrolysis, they all improve hydrolysis at some concentrations. The addition of acetovanillone or protocatechuic acid improved the hydrolysis of cellulose at low concentrations (0.05% and 0.005%, respectively), but inhibited the enzyme activity at much higher concentrations. The addition of increasing amounts of vanillin, vanillic acid, and guaiacol enhanced hydrolysis. In a similar manner, addition of both lignin and black liquor also increased the activity of cellulase with increasing their concentration. Observation by circular dichroism revealed a change in cellulase structure, indicating binding of the enzyme to some components of the black liquor. It appears that no deactivation of cellulase occurs due to the addition of either dissolved lignin or black Liquor, rather the interaction between the enzyme and lignin or black liquor results in an enhancement of hydrolysis.