An electrode comprising: a conducting substrate for detecting an electroactive agent; a catalytic agent held on the substrate for converting a non-electroactive agent to an electroactive agent by a catalytic process which consumes oxygen; and an oxygen reservoir, for releasing oxygen to feed the catalytic process, held within a polymer matrix on the substrate. The electrode allows for detection of materials such as glucose when using oxidase enzymes. The invention solves the problem of fluctuation or depletion of oxygen in the environment in which the measurements are being taken.