Mercury reduction and oxidation on ultrathin gold electrodes was investigated by simultaneous voltammetry and surface plasmon resonance (SPR). The angle dependencies of SPR signal were analyzed quantitatively. The results indicate partially reversible formation of amalgam with optical properties in between of optical properties of mercury and gold. Up to 12% content of mercury in the gold layer is suggested. (c) 2007 Elsevier B.V. All rights reserved.