From the double layer charge densities, obtained with the dropping mercury microelectrode technique, analysed for aqueous 1 M KF, 1 M KCl, and their mixtures, it is found that the thiourea (TU) adsorption is congruent if described at constant electrode potential and not at constant charge density. Application of some familiar isotherms is attempted and the consequences are discussed. The nature of the anion does not influence TU adsorption. On the contrary, Cl- adsorption is changed considerably by the presence of TU. A phase transition in the adsorbed phase seems to exist also at a relatively small surface excess of TU.