After a short introduction into the usage of organic barrier coatings and some related problems of evaluating coating performances, the principles of relaxation voltammetry (RV) measurements are given. RV is a new electrochemical technology in the time domain. The new features of this technology are high sensitivity and a relatively high sampling rate. This allows the measurement of the coating resistance R-C and of the coating capacity C-C even if good barrier coatings are used. RV offers a two-step data interpretation: first a very direct and easy accessible extraction of R-C and C-C. Second, a much more detailed interpretation of electrochemical processes in the coating is offered, i.e. dielectric relaxation, diffusion and charge transfer. In addition to the more classical interpretation of impedance spectroscopic data, RV permits a more detailed understanding of physical processes and their contribution to the relaxation of the previously charged coating surface. Evidence that the interpretation of barrier coatings by using only-RC elements is not suitable for the description of time-domain-measurements of immersed samples is given. The newly derived model based on RV-data is able to describe a huge number of coating materials under quite different aging conditions.