The dependence of the transmission properties of metal hole array prepared by a two-step molding process using porous alumina on the refractive index of the medium embedded in the holes was examined. A metal hole array membrane was set in a cell containing aqueous solutions with different refractive indices depending on their concentrations. The peak wavelength of the transmission spectra of the hole array showed a good linear relationship with the refractive index of the medium. Such unique optical properties will be useful in the application of the metal hole arrays to spectroelectrochemical analysis combined with electrochemical application. (C) 2004 The Electrochemical Society.