Gold nanoparticles on a heavily-doped Si substrate were synthesized through direct electroreduction of bulk AuCl4- ions. The optical properties of the nanoparticles were determined from ellipsometric analysis. The Lorentz-Drude model was used to represent the effective dielectric function of the gold nanoparticles. A prominent peak at similar to 1.7 eV is identified in the imaginary part epsilon(2), which is attributed to the surface plasmon resonance. There are also two minor peaks located at similar to 3.1 and similar to 4.2 eV, respectively, which are related to the interband transitions. The interband transitions are greatly suppressed, which could be due to the modified lattice structures of the nanoparticles. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3583646] All rights reserved.