A diffusion-kinetic model is presented for an enzyme-modified microcylinder electrode, where the enzyme reaction generates an electro-active product. Simple, approximate expressions are derived for the steady-state current in cases where the enzyme is immobilized in a metallically conducting, or a non-conducting matrix. The model is also extended to the chemical sensor case, of a conducting polymer without enzyme. The model is used to analyze steady-state signals for glucose produced by Pt-coated carbon fibres, on which glucose oxidase has been entrapped in poly(1,2-diaminobenzene).