The study of the adsorption/desorption mechanism of phosphate anions at Pt(111) in acidic solution of pH 4.3 and 0.8 was performed by the potential step method in order to reveal the kinetics of anion adsorption. The current-time curve due to phosphate adsorption/desorption showed various decay features, being dependent on the potential region. The rate of current decay depended on pH, being faster in a lower pH solution. Specific adsorption processes were analyzed by the Langmuir and Elovich adsorption equations and also in terms of a two-dimensional nucleation-growth mechanism in different adsorption/desorption regions. In the case of adsorption in 0.3 M phosphate buffer solution of pH 4.3, random adsorption without interaction following the Langmuir adsorption, takes place at low coverage, while random adsorption with repulsive force was observed at high coverage. In the desorption process, random desorption with repulsive force takes place at high coverage, and the repulsive force disappears where random adsorption without interaction takes place at medium coverage. When the surface coverage becomes further lower, the desorption mechanism changes dramatically into a two-dimensional nucleation-growth type, suggesting that an ordered adsorbate structure is formed after a rapid discharge process of anion adsorption.