Adsorption of organic compounds on electrodes involves the competition between organic molecules and the water molecules near the electrodes. From the thermodynamic analysis of electrocapillary curves, it is impossible to obtain surface excess of water. Therefore, another methods are required to know about the water molecules near the electrodes. In this monograph at first the adsorption of water molecules on the electrodes by various methods are reviewed. Adsorption of carboxylic acids on mercury electrodes are reviewed as in the last review (Sawamoto, 2006). Carboxylic acids described here are linear mono-carboxylic acids such as acetic acid, propionic acid and di-carboxylic acids such as oxalic acid, benzoic acid and related compounds.