Horseradish peroxidase has been proven effective in removing phenolic compounds in wastewater and additives such as polyethylene glycol have been found very effective in reducing the minimum enzyme close required. The effect of additives on horseradish peroxidase-catalyzed removal of phenol was investigated in this study. In the absence of additive, active enzyme is predominantly inactivated by the polymer product formed during rite reaction. The specific activity of horseradish peroxidase is higher due to the presence of additive. Experiments suggest that additives combine with the polymerization products formed during the reaction, because additives have a higher partition affinity with the polymer products than peroxidases. Most of the polymer product is coupled with additive so that less enzyme interacts with the polymer product. Horseradish peroxidase still combines with polymer products and becomes inactivated but at a much slower rate when additives are present. Consequently, the enzyme activity is protected by the additives.