Horseradish peroxidase-and tyrosinase-modified electrodes were constructed by entrapping the enzyme within an Eastman AQ 55D polymer matrix. The biosensors were used to detect their respective substrates (1 e organic peroxides and phenols) and inhibitors in polar orgamc solvents Horseradish peroxidase (HRP) inhibitors investigated mcluded thiourea, ethylenethiourea, mercaptoethanol, hydroxylamme and methyl isothiocyanate. Similarly, diethyldithiocarbamate was detected as a tyrosinase inhibitor Electrochemical methods such as cyclic voltammetry and steady-state amperometry, as well as spectroelectrochemistry, were used to characterise the analytical performance of these biosensors in organic solvents, namely acetomtnle, methanol, acetone, tetrahydrofuran, 2-propanol and 2-butanol. Operational parameters, such as the use o f a mediator, the effects of the chemical nature of the utilised mediator and its concentration, water content of the solvent and workmg potential were investigated and used to optimise the catalytic performance of the biosensors. The kinetic parameters for the sensors in the different organic media, such as the current as the substrate concentration approaches infinity (Imax), the apparent Michaelis-Menten constant (Km'), the apparent inhibition constant (K/) and Hill coefficients were determined. The differences in the values of these parameters were linked to the physico-chemical properties of the organic media, 1e solvent properties such as polarity, hydrophobicity, kinematic viscosity, dielectric constant, and solventenzyme mteractions. Finally, a glassy carbon electrode was modified with tyrosinase and a conductmg poly(l-vmylimidazole)-based osmium polymer. The ability o f this biosensor to function as a detector for high performance liquid chromatographic (HPLC) analysis of phenolic compounds such as p-ammophenol, phenol, p-cresol, catechol and pchlorophenol was characterised. The phenol sensor exhibited a 100 to 200-fold improvement in sensitivity and detection limit when compared to an ultraviolet-visible spectrophotometnc detector. The effects of operational parameters, such as the concentrations of the modifying compounds 1e the osmium polymer and the crosslinking compound, poly(ethylene glycol), the mobile phase composition and flow rate, the workmg potential and the biosensor’s short-term stability were investigated and optimised. The biosensor was used to measure phenol m an antiseptic cream, p-acetamidophenol (paracetamol) in a cold/flu relieving salt, and phenolic compounds in cigarette filter tips.