Nine phenol derivatives, p-coumaric acid (PC), vanillin (V), acetovanillone (AV), acetosyringone (AS), syringaldehyde (SA), coniferaldehyde (CLD), ferulic acid (FRC), sinapic acid (SNC), and sinapyl aldehyde (SLD) were assayed as laccase redox mediators in the biobleaching of kenaf and sisal pulps. As a general behaviour, the phenolic mediators increased the kappa number (KN) and reduced the brightness of pulps. In particular, these changes were found to depend in a linear manner on the energy of the highest occupied molecular orbital (E-HOMO) of the mediators. The phenolic mediator with the lowest E-HOMO (PC) led to the highest increase of KN and the lowest reduction of brightness. On the contrary, syringyl derivatives (i.e. SA) with high E-HOMO values caused small KN increases and significant losses of brightness. This behaviour was explained on the basis of a competition between grafting and polymerisation processes. The former basically affects KN, whereas the latter affects pulp brightness. (C) 2012 Elsevier Ltd. All rights reserved.