The diffusion coefficients of water and Cl- ion in epoxy coatings on aluminum alloys LY12 were investigated by electrochemical impedance spectroscopy (EIS). The key task in the present work remained in the model fitting of EIS data, from which coating capacitance was derived and was used to calculate water diffusion coefficient by the commonly used "capacitance method". Moreover, by the fitting analysis of EIS, appropriate equivalent electrical circuits (EEC) were proposed for different immersion stage. After that the initiation time for the appearance of each EEC was known. These times were correlated to the required times for penetrants arrival through the coating at metal interface. Then the diffusion coefficient was calculated from these characteristic required times in the case of Fickian diffusion. This novel method is defined as "required time method" in this work. It was shown that comparative values were obtained by using both the capacitance and required time methods for the water diffusion coefficient. Even more, the diffusion coefficient of Cl- ion calculated by required time method was very close to that reported in the literature ( measured by nuclear magnetic resonance (NMR) technique). (C) 2004 Kluwer Academic Publishers.