Stark effect on vibration frequencies of sulfate on Pt(111) electrode is studied by density functional theory calculations on a neutral surface model under a homogeneous external electric field. The calculations indicate that the Stark effect is dominated by the classical electrostatic interaction of the dipole moment of sulfate with the external electric field. The large potential-dependence of the S-O (uncoordinated) vibration mode, which was observed at 1250 cm(-1) by infrared reflection absorption spectroscopy (IRAS), is attributed to its large dynamic dipole moment normal to the surface. The large dynamic dipole moment significantly changes the equilibrium S-O (uncoordinated) bond distance in the external electric field, which results in the significant change in the vibration frequency. (C) 2013 Elsevier Ltd. All rights reserved.