Poly-aniline and o-methoxyaniline films can be obtained directly on an ultramicroelectrode through the anodic electropolymerization of the monomer through successive potential cycles. The potentiometric response depends on the nature of the anion incorporated in the course of the electropolymerization. At best, a linear relationship with pH is observed for 3 < pH < 9-10, with a slope of similar to 60 or 50 mV per pH unit for polyaniline and poly(o-methoxyaniline) ultramicroelectrodes, respectively. Such modified electrodes can be used to probe pH variations resulting from ion exchange in a thin Nafion film. (c) 2007 Published by Elsevier B.V.