The reactivities of amperometric horseradish peroxidase (HRP) electrodes in acetonitrile, methanol and acetone were studied by cyclic voltammetry. l,l-Dimethylferrocene (DMFc) was used as a soluble electron transfer mediator. The responses of the HRP electrode to butanone peroxide (BTP) as a substrate, and thiourea (THU) and ethylenethiourea (ETU) as inhibitors, were measured. Michaelis-Menten analyses of the calibration curves obtained for BTP, THU and ETU in the solvents were carried out to determine the apparent Michaelis-Menten constant (Kk), the limiting current (I,,,,,) and the apparent inhibition constant (Ki). The catalytic efficiency of the biosensor was determined as the ratio of the peak current controlled by the enzyme catalytic reduction of BTP to that controlled by the diffusion of DMFc to the electrode surface ( Ik/ld). The values of the Michaelis-Menten kinetic parameters and I,/(, were used to assess the performance of the peroxide sensor in the presence and absence of inhibitors. Our results indicate a decrease in sensor performance with increasing solvent hydrophilicity (methanol > acetonitrile > acetone). This has been attributed to the ability of the hydrophilic solvents to desorb essential water of hydration from the enzyme active site. This removal of water decreases the rate of enzymatic reaction and subsequently the catalytic performance of the sensor.