The surface potential kinetics of the nonionic surfactants tridecyldimethylphosphine oxide and dodecanoic acid at the air/water interface have been investigated by the vibrating plate method, An influence of the thickness of the solution layer on the surface potential and adsorption kinetics was observed. A mathematical solution for the case of diffusion controlled adsorption kinetics is presented. The experimental data were fitted for different models, As a result, the depth of the solution layer has to be taken into account in dynamic processes for small concentrations of these amphiphiles. This is important for strong surfactants. The fit parameters have reasonable values for the whole set of data by use of the derived equations. In the case of very low concentrations, a peculiar behavior in the surface potential kinetics during the initial adsorption time (t < 1 min) was found. We suppose this to be due to reorientation of the adsorbed surfactant molecules when the number of the molecules at the interface increases. This was previously observed by one of us on the basis of equilibrium surfactant adsorption isotherm analysis.