The adsorption and specific activities of various cellulases were evaluated during the hydrolysis of Avicel and Douglas fir-derived kraft and refiner mechanical pulps (RMP). Both the RMP and kraft pulps required higher enzyme loadings and longer hydrolysis times to achieve complete hydrolysis than did Avicel. Complete hydrolysis of a 2% kraft pulp required an enzyme loading of 60 FPU g(-1) cellulose while loadings as high as 750 FPU g(-1) cellulase did not result in complete hydrolysis of the RMP. Almost all of the protein originally added to the Avicel could be recovered in solution after complete hydrolysis of the substrate; however, only 50% of the initially added protein (40 FPU g(-1) cellulose) was recovered from the two pulps after 72 h incubation. During the hydrolysis of all three substrates, a change in the specific enzyme activities and ratios in the reaction filtrate were observed when compared to the initial enzyme preparation. The xylanase and endoglucanase activities were more closely associated to the recalcitrant Avicel residue while these activities were preferentially released into solution during hydrolysis of the kraft pulp. There was no significant difference in the distribution of the enzyme activities during the hydrolysis of the RMP. For all of the substrates, the filter paper activity was tightly associated with the insoluble fraction throughout the reaction.