The mechanism unterlying the chemo-enzymatic graft copolymerization of lignin with acrylic compounds was studied. When lignin sulfonate (LS) was incubated with the phenoloxidase laccase in the presence of a peroxide species (t-butylhydroperoxide, i.e. t-BHP), the decrease of the phenolic groups was significantly higher than when LS was incubated with laccase or t-BHP, separately. The enzymatic oxidation of the phenolic groups did lead to the formation of quinones however, the number of quinone groups formed in LS in the presence of t-BHP? was not significantly higher than in its absence. Still, a certain proportion of the quinones formed in the presence of t-BHP appeared to be further oxidized resulting in an opening of the aromatic rings. In addition, the rate of LS polymerization, i.e. its increase in (M) over bar (w) instigated by laccase, was significantly reduced in the presence of t-BHP even though the activity of laccase was only slightly affected by the peroxide. Incubation of LS with laccase prior to the grafting reaction resulted in a significant increase of die spin concentration as detected in the EPR spectra. In spite of a high steady state concentration of radicals, the generation rate of these radicals in pre-incubated LS was lower than in unpretreated LS. Thus, the polymer yields obtained with pre-incubated LS were lower than those obtained with unpretreated LS.