Composite electrodes comprising Ketjenblack (carbon black) in Kel-F [poly(chlorotrifluoroethylene)] were prepared by dispersing carbon black in a solution of Kel-F in carbon tetrachloride at 150-degrees-C under autologous pressure. (p-Xylene was used as an alternative solvent for composite materials containing silver or mercury, which react violently with hot CCl4.) After removal of the solvent, the dry mixture was pressure molded to give electrodes that had low background currents in aqueous and nonaqueous electrolyte solutions : Cobalt phthalocyanine-modified electrodes (Kel-CoPc) were prepared by adsorbing cobalt phthalocyanine onto carbon black particles before preparing electrode materials as described above. Electrochemical pretreatment gave composite electrodes that yielded low background currents and low overpotentials for hexammine ruthenium (III) chloride, hexacyanoferrate(III), and ferrocene. Electrochemically pretreated Kel-CoPc electrodes performed well in electrochemical detection of L-cysteine in high pressure liquid chromatography.