Horseradish peroxidase (HRP) catalyses the oxidation of toxic aromatic compounds, especially phenols, in the presence of hydrogen peroxide. Reaction products polymerise to form insoluble precipitates which readily separate from aqueous solution, unlike their monomeric precursors. High-temperature phenol-containing gas liquors (produced from coal conversion processes) or effluent from bleach plants of kraft mills can substantially affect the stability of enzymes such as HRP and thus their oxidation capabilities. Apparent inactivation of peroxidase during high temperature polymerisation reactions is mainly due to unfolding of the protein backbone. The catalytic lifetime of HRP at high temperatures can be extended by chemical modification of lysine epsilon-amino groups using succinimides. The bifunctional, ethylene glycol bis-succinimidyl succinate (EG-NHS) and the monofunctional, acetic acid N-hydroxysuccinimide ester (AA-NHS) were used. The extent of stabilisation is dependent on the nature and concentration of the reagent used. The optimum pH for phenol removal is 9.0 (8.0 for 4-chlorophenol) for both native and modified forms of the enzyme; the optimum molar ratio of hydrogen peroxide and phenolic substrate is around 1.0. The effects of peroxide and enzyme concentration on the polymerisation reaction were investigated. HRP derivatives significantly reduced the oxidation reaction time at 70 degrees C.