The mode of action of several Trichoderma reesei cellulase preparations was investigated in relation to their effect on the crystallinity and degree of polymerization of a fully bleached softwood kraft pulp. One complete (Celluclast 1.5L, Novo Industri) and six modified cellulases (Rohm Enzyme Finland Oy, former Primalco Ltd Biotec) were used tinder a relatively low enzyme loading of 40 mg protein g(-1) cellulose in a reaction mixture in which an excess of exogenous cellobiase activity was added (Novozym 188). This enzyme lending was used To assess the mode of action of the enzymes under both non-saturating and non-inhibitory conditions. The modified cellulase preparations were obtained from T. reesei by deleting the following components of the cellulase system: EG I and/or EG II or CBH I and/or CBH II. Characterization of hydrolysates by gel permeation chromatography (GPC) showed that short incubation times of 4 h with CBH I-deficient enzymes (CBH I-and CBH I/II-mutants) resulted in the highest shifts in the molecular weight distribution of cellulose. Compared to EG-deficient enzymes (EG I; EG II- and EG I/II-mutants), the lack of CBH II resulted in equivalent changes in both polydispersities and average degrees of polymerisation of cellulose. This suggested that the role of CBH II in hydrolysis was similar to that of both endoglucanases I and II. The effectiveness of all mutant enzymes in hydrolysing cellulose was much lower than that obtained when a complete cellulase system such as Celluclast 1.5L was used. Our results also indicated that the lack of CBH I was very detrimental to hydrolysis and depolymerization of cellulose and that EG I has a greater effect on the substrate than EG II. However, there were no detectable changes in the degree of crystallinity of partially hydrolysed pulps, regardless of the enzyme system used.