Environmental pressure has led the pulp and paper industry to develop new technologies in order to reduce or suppress the presence of various pollutants in effluents from bleaching plants. One of the choices for this purpose is enzyme-based biotechnology. This study deals with the effect of using a xylanase-based enzymatic pretreatment, in a TCF (Totally Chlorine Free) sequence, on the properties of the resulting paper pulps. The hexenuronic acid content in the pulp and the physical properties of the paper were also studied. The performance of the xylanase was analysed through kinetic studies on ozone bleaching. The enzymatic pretreatment results in easier bleaching and delignification of the pulp, causing a bleach-boosting effect. The decreased consumption of reagent is related to a decreased content of hexenuronic groups. The physical properties of the treated pulp are similar to those of untreated pulps. Cellulose degradation, delignification and chromophores' removal show first-order kinetics. Enzyme pretreatment leads to differences between the kinetic constants of cellulose degradation and chromophores' removal, due to an increased accessibility to bleaching agents. The Xylanase treatment leads to a lower floor kappa number (IKinfinity) during the ozone stage. (C) 2003 Society of Chemical Industry.