Activation of fibres by radical formation is the first step when aiming at oxidative coupling of new functional groups on the fibre bound lignin. In this work, factors affecting the amount of phenoxy radicals created to unbleached TMP, CTMP, softwood kraft and hardwood kraft pulp fibres in the laccase catalysed oxidation were determined by EPR. Laccase was able to catalyse the oxidation of all the pulps studied. The reactivity of the pulp was found to be affected by both the physical accessibility of lignin in the fibres and the chemistry of the surface lignin accessible to laccase. Laccase dosage, use of extra oxygen in the laccase catalysed radicalization reaction, treatment time and also the amount and type of low-molecular weight compounds (LMWC) present in the pulp were all found to contribute to the radical content of pulp fibres measured after the enzymatic reaction. It could not been excluded that two types of reactions take place during the radical formation in fibres. Within the fibre matrix there may be both fibre material bound and soluble lignin fragments differing with respect to accessibility, molecular weight or chemical structure which can be radicalized at various rates, and the formed radicals may also undergo cross-coupling reactions reducing the amount of the total radicals. (C) 2009 Elsevier Inc. All rights reserved.