Redox-modified oligosiloxanes were prepared from an Si-60 oligo(hydrogen siloxane) and allyl-terminated oligo(ethyleneglycol) (OEG) esters of 3-carboxy-2,2,5,5-tetramethyl-Delta 3-pyrroline-1-oxyl (PROXYL-COOH). Glassy carbon disk electrodes coated with the modified siloxane films were obtained by spin coating with film thicknesses between 10 nm and 15 mu m and analyzed by cyclic voltammetry, chronoamperometry, and rotating disk voltammetry in acetonitrile and water. Due to the OEG content, the siloxane matrix offers a high. diffusivity with respect to the mobility of redox centers and solute species. The PROXYL-mediated catalysis of the oxidation of 4-methoxybenzyl alcohol (PMBOH) to anisaldehyde was studied at spin-coated glassy carbon disk electrodes by cyclic voltammetry in acetonitrile with lutidine as a base and in buffered aqueous solution at pH = 7. The catalytic efficiency is 10-fold higher in the latter medium. In controlled potential microelectrolyses, current densities up to 15 mA/cm(2) and catalyst turnovers up to 30 000 were determined at ca. 0.1 mmol/cm(2) substrate conversion. Crucial factors of film performance are thickness and homogeneity, which are still difficult to control at larger area carbon felt electrodes.