The characteristics of an electronic differentiator that facilitates continuous monitoring of changes in the oxygen concentration measured by a polarographic oxygen electrode are described. In addition, some properties of the electrode membranes that increase the sensitivity of the oxygen-sensing electrode are also described. These modifications greatly improve the accuracy and sensitivity of the biological oxygen monitor for use in kinetic studies of complex enzyme systems such as soybean lipoxygenase or prostaglandin cyclooxygenase. The electronic differentiator also may be used in conjunction with other ratemonitoring instruments such as spectrophotometers and pH meters for accurate quantitation of rapidly changing rates.