The adsorption of cinchonidine (CD) on polycrystalline platinum surfaces in 0.5 mol dm(-3) H2SO4 has been investigated. It has been found that adsorption is irreversible, desorption takes place at relatively high potentials simultaneously with the oxidation of CD molecules. According to our results CD covers only a part of the Pt surface and maximal coverage is influenced by the surface structure. Some assumptions for the site requirement of an adsorbed CD molecule have also been made.