Due to its inherent stability, ease of handling and availability, horseradish peroxidase (HRP) will continue to be used more extensively in analytical and industrial situations. It is often used as a model for other peroxidases. Developments in the fields of protein stabilisation and biosensor construction are discussed in Chapter one; also, various enzymatic methods for treating phenolic effluents are reviewed. The effects of chemical modifiers on native HRP were investigated (Chapter two). Homobifunctional crosslinkers specific for lysine residues were employed. No loss of enzyme activity occured on reaction with such Nhydroxysuccinimide (NHS) compounds. Derivative forms of HRP displayed greater thermostability and a greater tolerance of water-miscible organic solvents. Enhanced resistance towards dénaturants was noted. Structural changes in the vicinity of the heme of HRP derivatives were studied by UV/Visible spectrophotometry and fluorimetry. The extent of modification on HRP’s six lysines has been determined. The NHS derivatives of HRP have also been employed in the removal of phenols from aqueous solution (Chapter three). HRP catalyses the oxidation of toxic aromatic compounds in the presence of hydrogen peroxide. Reaction products polymerise to form high molecular weight materials which can be easily separated from aqueous solution. Modified peroxidases displayed greater removal efficiencies of phenols compared to the native enzyme over a wide range of reaction conditions, including high temperatures. For some pollutants, the efficiency of removal is high. Native HRP has also been used in the development of a biosensor for the selective determination of uric acid (Chapter four). The sensor was found to function efficiently without the necessity for an electron transfer mediator. The mechanism of the sensor’s response was thought to be due to direct electron transfer from the electrode to HRP. A monomer, o-aminophenol, which was electrodeposited at the working surface of the electrode, was found to protect the biocomponents from interferences and fouling. The sensor was incorporated into a flow injection system for the quantification of uric acid in human serum. Recoveries compared favourably with a standard spectrophotometric method.