Methanol vapor-induced membranous changes in a cast-coated Nafion thin film were studied through current-voltage (I-V) characteristics with an interdigitated microarray (IDA) electrode and atomic force microscopy (AFM). The obtained I-V curves showed that the as-prepared Naflon film was stable under humidified nitrogen gas; however, the I-V profile dramatically changed with exposure to methanol vapor. Next, the morphology of the film was compared before and after methanol exposure with AFM images. On the basis of our observations, we found that the as-prepared film had an irregularly complicated microstructure, whereas the structure became homogeneous in appearance after 30 min of exposure to methanol gas. The alternating-current conductivity data, showing almost the same magnitude before and after exposure, strongly suggested that the I-V profile shift was based on a change in an electrode reaction mechanism induced by a change in the junction at the Nafion/IDA electrode interface. Furthermore, the methanol vapor-pre-exposed Nafion was stable for further exposure to methanol vapor, water vapor, or both. With the stabilized film used in combination with the IDA electrode, a reversible change in the magnitude of the current was observed when the methanol/water vapor ratio was varied. This indicated that the electrode reaction had good reproducibility after the treatment.