The aim of this paper was to study the adsorption of L-DOPA on HMDE using experimental data obtained from measurements of the differential capacitance of the double layer, the zero charge potential and the surface tension at this potential. The experimental data presented in this paper on the adsorption of L-DOPA on a mercury electrode at different concentrations and different pH values of the base electrolyte can expand the knowledge on the properties of the mixed adsorption layer formed by the tested organic substance and the components of the base electrolyte. The dependence of the differential capacity of the double layer on frequency observed in the studied systems indicates that adsorption equilibrium is not established during the lifetime of a single drop. Since the components of the base electrolyte: CH₃COO- ions and CH₃COOH molecules adsorb on the electrode, the lack of adsorption equilibrium in the presence of L-DOPA may be the result of competitive adsorption of ions and adsorbate molecules present in the solution.