The residual toxicities of aqueous solutions of phenol and substituted phenols were investigated following polymerization under the catalytic action of soybean peroxidase (SBP) and horseradish peroxidase (HRP) enzymes. The treated mixtures obtained from the enzymatic polymerization of these phenols were usually significantly more toxic than expected, and in several cases, the residual toxicity exceeded the initial toxicity of the solution of untreated parent compound. However, this residual toxicity tended to be lower when combinations of these phenols were copolymerized. The decrease in toxicity was attributed to the different polymeric products which form as a result of a cross-coupling between products of the enzyme-catalysed oxidation of parent phenols. The residual toxicities obtained using either SEP or HRP were not significantly different in most cases.