The diffusing behavior of water through alkyd coatings with different pigment volume concentration (PVC) has been studied by EIS and SEM techniques. The corresponding diffusion coefficients were also calculated. The experimental results showed that, in 0.5 mol· L－ 1 NaCl solution, the diffusion process of water through alkyd coatings with different PVC followed the ideal-Fick′ s law in the initial period.Within the PVC value concerned, the diffusion coefficients initially decreased with the increase of PVC parameter, reaching a minimum value at 60％ , which is the CPVC of micaceous iron oxide, and then increased with the PVC increase. When the PVC was less than CPVC, the barrier property produced by the pigment was the main factor that affected the water diffusion behavior. However, the microstructure of the alkyd coatings played an important role when the PVC was greater than CPVC.
