Enzymes can be immobilized near electrode surfaces by trapping them in a crosslinked polymer. Gregg and Heller (1) introduced an mteresting extension of this technique by incorporating the polymer into the sensortransduction mechanism. The strategy was to attach redox complexes to the polymer backbone and use the resultant redox polymer to mediate electron transfer between the immobilized enzymes and the substrate electrode. The principles behind the operation of these enzyme electrodes are essentrally identical to those behind sensors that use soluble mediators. The main difference IS that the redox functionallttes pendant to the crosslinked polymer cannot participate in molecular diffusion, Rather, the electron transport process occurs by a series of self-exchange reactions between neighboring redox centers, a process referred to as electron hopping (2).