The adsorption of OH- ions on the Pt(111) plane has been studied by fast cyclic voltammetry in sodium hydroxide solutions (0.03 to 1 M), under quasi-equilibrium and Tafel approximation conditions. It was shown that the OH- ion adsorption is an electrosorption process with one electron exchanged between an OH- ion and the platinum surface. The electrosorption process follows the Frumkin adsorption isotherm with low intensity repulsive interactions of the adsorbed species (f=2-3). The estimated values of the standard electrochemical rate constant (k degrees = 5.6 x 10(-4) cm s(-1)) and the standard exchange current density (j = 5.45 x 10(-2) A cm(-2)) indicate a rather fast electrochemical process.