Class fiber reinforced polyester composites are widely used in marine applications where they may undergo chemical and physical aging. For this kind of laminates, the durability is governed mainly by the matrix degradation. The mechanical property reduction is not induced only by hydrolytic aging but rather by the formation of cracks or blisters. That is the reason why it became of major interest to understand the microcrack nucleation mechanism in neat matrices, to study the crack propagation, and to evaluate the incidence of cracking on the tensile property loss. We first put forward a nucleation mechanism based on polyester-organic molecule phase separation, and drew a simple kinetic model to take into account the existence of a crack induction time that is dependent on temperature and matrix structure. Crack propagation results from the build up of an osmotic pressure in microcavities, which is proportional to solute concentration. This second step has been studied in terms of crack density evolution and crack propagation rate. Finally, there is a good correlation between tensile mechanical properties variation and crack parameters.