The behaviour of unsaturated polyester to water at room temperature is evaluated by determining the kinetics of absorption of water. As the process of absorption is controlled by diffusion, two parameters can thus characterise these kinetics, with the diffusivity and the amount of water absorbed at equilibrium. On the other hand, the state of cure of a thermosetting resin is the main characteristic of the resin. The state of cure is expressed by the extent of heat evolved by the cure reaction in the mould as a fraction of the total cure enthalpy. Correlations are made between the diffusivity and amount of water absorbed at 60°C and the values of the state of cure for various unsaturated polyester resins.