Water plays a pivotal role in several electrocatalytic reactions. In the electro-oxidation of small organic molecules, water can for instance assist the oxidation of adsorbed species or inhibit the reaction through the oxidation of surface sites. This paper is the third in a series of studies aiming at evaluating the impact of water concentration on the electro-oxidation of small organic molecules on polycrystalline platinum. The study was performed in phosphoric acid electrolyte, and the water concentration was varied from 14 to 50 mol L-1. Voltammetric profiles and potential oscillations were studied at distinct [H2O]/[H3PO4] ratios and constant concentration of formic acid. There is a diminution in the catalytic activity accompanying the decrease in the water concentration. The removal of carbon monoxide demands oxygenated species and can proceed in the absence of water for ethanol and methanol, but not for formic acid. Under oscillatory regime, the impact of water concentration, it is seen that higher water concentration implies higher oscillation frequencies, shorter and less stable time-series, and sharper transitions from low to high potential, where the adsorbed carbon monoxide is oxidized by adsorbed oxygenated species. Results are discussed in connection with the voltammetric study and also compared with other parent systems.