The reaction of the monomeric lignin model compound guaiacol and the beta-O-4-type dimer erol (1-(4-hydroxy-3-methoxyphenyl)-2(2-methoxyphenoxy)-propane-1,3-diol with laccase from Trametes hirsuta was studied in the presence of the mediator ABTS (2,2'-azino-di[3ethylbenzothiazoline-6-sulfonic acid]). The product mixtures were analyzed by means of aqueous-phase size exclusion chromatography (SEC) with 50 mM NaOH as eluent. Interestingly, in the laccase-catalyzed reaction with both substrates, the mediator not only functioned as an electron carrier but underwent coupling reactions with the substrate to give polymeric coupling products. The molecular weight of these copolymeric products was significantly higher than the molecular weight of products obtained without ABTS. After ultrafiltration, 33% and 21% of the initially applied ABTS could be found in the polymeric product fraction for the substrates guaiacol and erol, respectively, on the basis of nitrogen analysis. When ABTS was added to substrates after full laccase-catalyzed polymerization, the reaction proceeded toward higher molecular weights.