According to the nature of the system, superficial transitions may involve one, two or three distinct particles. Since transition potentials are generally very accurately defined, they can be used to derive information about the various factors acting on adsorbability. The kinetic study of transition processes shows that their rate is controlled by the creation and expansion of nuclei. However, the kinetic parameters differ notably in function of the nature of the system and the amount of electrical energy required.The adsorption processes are usually characterized by the gradual variation of the components of superficial excess, when the electrode potential is changed, for a fixed concentration of surfactant, or vice-versa.However, as it has been pointed out by Frumkin [l], if the adsorbability is enhanced by lateral attractive shortrange forces, the isotherm exhibits a typical S-shape which presents ,,forbidden" regions. This behavior is reflected by sudden steps (,,transitions") in the experimental curves relating the superficial excesses to the potential or the concentration.A classical example is afforded by the adsorption of pyridine and its derivates at the mercury-electrolyte interface. The transition process may occur within a potential gap as little as a fraction of a millivolt, for which the surfactant coverage steps from zero to unity [2]. This structural change brings a break in the electrocapillary curve, a discontinuity in the electrode charge density, a step (usually combined to a narrow peak) in the differential capacity and also a sudden variation in the rate of any concomitant faradaic processes.Measurements of the interfacial tension remain essential for assessing the local composition, but are rather insensitive indicators for the existence of transitions. I n fact, most of the known systems have been detected by discontinuities in their inhibiting action.Superficial transitions are more frequent than it is generally thought. They may reflect various types of local changes which can be classified a t least into three distinct categories : 1) "Homoparticular transitions" involve only one kind of moieties. At sufficiently large bulk concentrations, the electrode is covered by a compact monolayer stabilized by lateral interaction. It consists of a two-dimensional cristallographic array of particles which assume a definite orientation. When one acts upon the system by changing the electrode potential or the composition, another structure suddenly prevails. The role of the potential is related * Paper presented to the meeting of the GDCh-Working Group ,,Applied Electrochemistry" at Julich (Germany) at October 13-15, 1971. to its field effect on the orientation, whereas the surfactant concentration controls the range of the closest-compact configuration.A striking example is afforded by the /3-quinoline, which can assume three different structures amongst which the two perpendicular orientations are mutually exclusive. That field-induced dipole switching is the determining factor is substantiated b...