A new type of spectroelectrochemical sensor that demonstrates three modes of selectivity (electrochemistry, spectroscopy, and selective partitioning) is demonstrated. The sensor consists of an optically transparent electrode (OTE) coated with a selective film. Sensing is based on the change in the attenuation of light passing through the OTE that accompanies an electrochemical reaction of the analyte at the electrode surface. Thus, for an analyte to be detected, it must partition into the selective coating and be electrolyzed at the potential applied to the electrode, and either the analyte or its electrolysis product must absorb light at the wavelength chosen. Selectivity for the analyte relative to other solution components is obtained by choice of coating material, electrolysis potential, and wavelength for optical monitoring. The sensor concept is demonstrated with an OTE consisting of an indium−tin oxide coating on glass that has been over-coated with a sol−gel-derived charge-selective thin film. Att...