The adsorption kinetics of octanoic acid at the mercury/electrolyte interface was investigated by means of capacity-time transients. For low concentrations of octanoic acid and for the short-term region, the transients can be theoretically well described with a diffusion-controlled adsorption process according to the Delahay-Trachtenberg law. The long-term transients for higher concentrations of octanoic acid exhibit an increase of the capacity with time, which can be modeled with an adsorption-controlled replacement of a condensed adsorption state by a hemimicelle adsorption state. Criteria are derived for which an increase of the double-layer capacity with time can be expected.