Adsorption of 2-, 3- and 4-fluorobenzoic acids on an Au(100) electrode in 0.1 M HClO4 have been studied using in situ reflection adsorption IR spectroscopy and cyclic voltammetry. It is found that for the fluorobenzoic acids the vertically adsorbed benzoate with both oxygen atoms oriented toward the metal surface is present at positive potentials, and a new adsorbed state due to this Vertical orientation appears around 0.75 V. The OCO symmetrical stretch band corresponding to the new adsorbed state is observed at a higher frequency than that of the original band. However, the bands due to ring stretches display no change at that potential. In the voltammetry, broad and sharp reversible peaks corresponding to both the adsorbed states are observed. This indicates that the new band should be associated with some interaction of the COO- group and the hydrated water. (C) 2000 Elsevier Science Ltd. All rights reserved.