The present paper is the first of a series dedicated to the interfacial behaviour of several surfactants of the quinoline group, respectively in the absence and in the presence of coadsorbed anions. The present communication deals with the adsorption of quinoline (Q) itself at the mercury—water interface, in a medium such that coadsorption can be considered as negligible. Study of equilibrium adsorption has been based on measurements of interfacial tensions, charge densities and differential capacities as a function of the potential and the Q concentration. The experimental isotherms have been analysed with the help of Esin-Markov plots. In the range of sufficiently negative potentials, a compact film of invariant structure is observed, which remains stable up to a sharply defined cathodic desorption potential. At less negative potentials, the Esin-Markov plots, as well as the capacity—concentration—potential map, indicate that three different superficial structures may prevail, according to the domain of potential and concentration:o(1)water with dilute Q molecules lying flat;(2)a mixed aqueous layer which embodies both flat and standing Q molecules;(3)a film characterized by abnormally large values of the superficial excess, reflecting the presence of randomly distributed multilayered clusters. No clear evidence has been found for a stable compact layer of molecules lying flat. As can be expected, surface—pressure plots are non-congruent, in view of the variety of orientation and lateral forces involved with potential or concentration changes. Some data are presented which will be made fully explicit at a later stage (Parts IV and V of this series), where the various surfactants will be compared in terms of their molecular structure and dipole orientation. water with dilute Q molecules lying flat; a mixed aqueous layer which embodies both flat and standing Q molecules; a film characterized by abnormally large values of the superficial excess, reflecting the presence of randomly distributed multilayered clusters. No clear evidence has been found for a stable compact layer of molecules lying flat. As can be expected, surface—pressure plots are non-congruent, in view of the variety of orientation and lateral forces involved with potential or concentration changes. Some data are presented which will be made fully explicit at a later stage (Parts IV and V of this series), where the various surfactants will be compared in terms of their molecular structure and dipole orientation.