An experimental method is proposed to determine in a quantitative way the through-coating porosity of thin ceramic coatings deposited on metallic substrates. The method is based on a coulometric measurement performed during potentiodynamic polarization of a coated sample. Since coating and substrate materials may interact electrochemically, a standard addition procedure to derive the porosity index has been worked out. The porosity of different TiN-based physically vapour-deposited coatings has been compared. The porosity values obtained are more realistic in comparison with those obtained by other methods.