Adsorption kinetics of small organic molecules was studied by ac voltammetry with mercury ultramicroelectrodes in the frequency range 0.22-6.25 MHz. It was found that the activation energy of the adsorption process can be expressed as a linear combination of the electrical component of the standard free energy of adsorption (a major contribution) and the energy of lateral interactions (a minor contribution). The results suggest that, during the adsorption process, work against an electrical field must be performed before excess energy (due to the change in the hydrophobic interactions) is released. The final state is reached by a rotational and translational motion of the surfactant molecule. At the zero charge potential the rate constant reaches the maximum value of (4.6 +/- 0.3) x 10(9) s(-1). This value indicates that the viscosity of interfacial water at mercury near the zero charge potential is about 1.7 times higher than the bulk viscosity.