A surface plasmon resonance sensing device has been constructed, in which excitation of a surface plasmon on a metal film is achieved by coupling from light propagating in an optical waveguide. The metal film cladding has been used as an electrode surface and it is proposed that these structures may be applied as novel optoelectrochemical sensors. Specific cationic adsorption has been monitored in potential regions where the metal free-electron contribution is small, and the sensitivity of this sensing technique has been deduced by comparison with ellipsometric measurements. The interfacial model used to analyse the ellipsometric results has been tested by calculating coverage by water in the water + ethylene glycol mixture employed in the surface plasmon device. The high sensitivity of this technique to adsorbed films is found to be greater than that predicted by the theoretical model described, and reasons for this discrepancy are discussed.