This specification discloses sensor electrodes, and their combination with reference electrodes; the manufacture of such electrodes; apparatus utilising such electrodes; and electrical circuitry into which such electrodes can be incorporated, specifically the establishment of certain design criteria in the production of such electrodes for lay, or clinic, use. The electrodes can be used in an invasive probe (i.e. one which enters body tissue to contact a body fluid such as whole blood or subcutaneous tissue fluid) or as part of an external test upon a withdrawn sample (using a syringe) or upon an expressed sample (e.g. using a needle-prick device). In the specification claims, sensor means for selective detection, measurement or monitoring of a given dissolved substrate in a mixture of dissolved substrates, comprising:- (a) an area 4 of first electrode material comprising an enzyme catalytic of the said substrate and a mediator compound to transfer charge to the electrode when the enzyme is catalytically active, adjacent to but non-contiguous with; (b) an area 5 of reference electrode material; both electrodes being of small dimension, and extending as or supported on an elongate member to facilitate manipulation before or during contact with live tissue or a small withdrawn sample of body fluid.