An effluent containing soluble lignin fragments was treated with potato-polyphenoloxidases (PPO) or horseradish peroxidase/hydrogen peroxide system (HRP/H2O2). In both cases the reaction was evidenced by the information of a brown precipitate that was a consequence of the polymerization of lignin fragments. The effect of reaction time, pH, and amount of soluble lignin per unit of enzyme activity on the insolubilization yield was evaluated for PPO-initiated reactions. For HRP-initiated reactions, the amount of H2O2 per unit of enzyme activity was also evaluated. Mathematical models were calculated to predict the insolubilization yield as a function of the significant variables. Based on these models, the insolubilization reaction was optimized and reached:maximal values of ca. 50% in both reaction systems. Higher insolubilization yields were not achieved. Chemical characterization of the soluble lignin fragments indicated that the insolubilization yield could not be improved, because the lignin fragments had limited amounts df free phenolic substructures available for the initial steps of the polymerization reaction. (C) 2000 Elsevier Science Inc. All rights reserved.