The diffusion of solvents from a coating layer and their evaporation at the surface of the coating film during the curing process of the coating are investigated by bulk acoustic wave (BAW) admittance analysis and can be expressed as change in the relative diffusion rate (J). The porosity and permeability of the coating film are related to the curing degree (eta) The changes in J and the values of J(s) (the relative diffusion rate at the surface dry time, t(s)), Delta t(p) (the time difference between the surface dry time t(s) and the turning point time t(p) when the diffusion behaviour changes), and eta can provide much useful information on selecting solvents and optimizing coating ingredients in the preparation of coatings. The effects of the coating film thickness, solvent content of the coating liquid, and the volatility of the solvent on the diffusion of solvent from the film are also discussed.