Adsorption of carboxylic acids from liquid solutions on gold was investigated by electrochemical experiments including voltammetric measurements and quartz crystal microgravimetry. The adsorption was found to occur at gold surfaces held at relatively high electrical potentials, and the adsorption step was associated with anodic currents. From the experimental evidences, we propose that the carboxylic acid molecules adsorb on a gold surface through an anodic reaction that is analogous to the previously studied adsorption reaction of organic sulfur compounds forming self-assembled monolayers. The potentials at which the carboxylic acids adsorb to appreciable extents were much higher than the potentials at which thiols start to adsorb. In all of the adsorption reactions of thiols, disulfides, and carboxylic acids, oxidation of the metal surface, assisted by the adsorbate−metal interaction, appears to be a common requirement for the adsorption.