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The diffusion behaviour of water through alkyd coatings with micaceous iron oxide pigment at different pigment volume concentration (PVC) was studied by EIS and SEM techniques. The diffusion coefficients of water through the coatings were also calculated. The experimental results showed that, in the initial diffusion period, the diffusion processes of water through alkyd coatings immersed in 0.5 M NaCl followed the Fick's second law. Within the PVC values which have been studied, the diffusion coefficients of water diffusing through alkyd coatings initially decreased with increasing PVC, reaching a minimum value at 60%, and then increased with increasing PVC. When PVC was less than 60%, the barrier property of pigment was the main factor affecting the water diffusion behaviour. However, the coatings' microstructure played an important role when PVC was greater than 60%. (C) 2002 Elsevier Science Ltd. All rights reserved.