Microarray electrodes coated with conducting polymers work as chemical sensing devices [l-6] based on a reversible change in electronic conductivity of the polymers in response to redox species [3-51 or a change in pH [4,6]. When an appropriate enzyme is immobilized into the conducting polymer, the chemical sensor responds only to a specific biomolecule [7-111. Since, with polymers such as polypyrrole and polyaniline, these conductivities can be altered by several orders of magnitude by changing pH values and potentials [ll, it is possible to obtain switching responses to chemicals and substrates for enzymes. The formation of thin polymer films at microarray electrodes is a key technique in the fabrication of conductometric devices because rapid and clear responses are obtained with thinner conducting polymer films [6,9]. In previous papers [12,13] we have described the formation of ultrathin polypyrrole at microarray electrodes, taking advantage of the effective lateral growth of electrosynthesized polypyrrole along glass substrates pretreated hydrophobically with n-alkylsilane reagents. Since the polymerization of pyrrole proceeds selectively at the hydrophobic surface, the partial pretreatment of the glass substrate provides a method of micropatterning with polypyrrole. We also reported some results indicating that the addition of dodecylsulfate (DS-) further enhances the effective lateral growth of polypyrrole along the hydrophobic surface. We report here additional results on the “amplification effect” of DS-, which is important for the practical application of our method to the fabrication of ultrathin and patterned conducting polymers on an insulating substrate.