Adsorption of isoquinoline at a Au(111) single-crystal electrode has been investigated using chronocoulometry, cyclic voltammetry, and phase-sensitive ac voltammetry. The experiments were performed in pH 10.7 solutions in order to extend the double layer region to more negative potentials. The adsorption parameters, such as the film pressure, Gibbs surface excess, Gibbs energy of adsorption, and electrosorption valency, were determined as functions of the electrode potential. Adsorption of isoquinoline has the character of a weak chemisorption. Two limiting surface coordinations of the isoquinoline molecule to the Au(111) surface were identified : the flat pi-bonded coordination at negative potentials and the vertical N-bonded orientation at positive polarizations. The transition from the flat to the vertical orientation has a gradual character and proceeds through a series of intermediate states.