Potential-induced phase transition of trimesic acid (TMA, C6H3(COOH)(3)) on a Au(111) electrode was investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry. A pair of peaks, ascribed to structural transformation was observed in the cyclic voltammograms (CVs) of Au(111) in 0.1 M HClO4 + 0.1 mM TMA. The magnitude of the peaks decreased with increasing solution pH. On the basis of highresolution STM images in 0.1 M HClO4, it is proposed that TMA molecules assume a flat-lying and a vertical orientation at 0.25 and 0.85 V, respectively. The coexistence of the two orientations was observed at 0.65 V. The discovered adlayer phases could be represented by (4x4), (5x2root3), and (2root3x 4root3) or (7x2root3) structures. Intriguingly, it is found that TMA molecules bind on Au substrate with one and two deprotonated carboxyl groups with potential shifting, even though the molecules take the same vertical orientation. On the basis of the STM observation, structural models for various adlayers are proposed. Comparing the STM results with those of cyclic voltammetry, we conclude that the potential- induced phase transitions in 0.1 M HClO4 solution result from the interfacial deprotonation/protonation of the TMA molecules.