Kinetic equations are presented for the adsorption of a surfactant from a liquid bulk phase to a liquid/liquid or air/liquid interface, for a situation where diffusion equilibrium between bulk and subsurface is established, and consequently the transfer of surfactant from the subsurface to the surface is the rate-determining step in the adsorption process. These equations are obtained in away very similar to that by which the Butler-Volmer equations are obtained in electrode kinetics, where also the reaction rate is the rate-determining step. Essential for both is that the standard free energy of the activated complex is split into a chemical part and a part which depends on the external force field (surface pressure for adsorption, electrical potential for transfer controlled electrochemical reactions), This part of the standard free energy of the activated complex is a fraction of the force-held-dependent part of the adsorption standard free energy, This reasoning leads f.i. to the Langmuir kinetic equation for the adsorption process, This Langmuir kinetic equation is extended for regular solution behavior and, for the case of an indifferent electrolyte, added to the surfactant system. These equations are in agreement with previous experimental observations, except for the regular system where experimental data are still lacking.