The diffusion of OH−-ions and H2 formed during cathodic protection through a paint film is studied. The diffusion equation is solved for non-stationary conditions and from this the steady state is also derived. It is shown that under usual operating conditions of cathodic protection of ships the stationary state is reached in about 15 days. The danger of paint deterioration through high alkalinity and blistering is found to increase with current density and paint film thickness and with decreasing diffusion coefficient in the paint. For practical calculations a better knowledge of diffusion coefficients in paints is needed.