Several nonreinforced and reinforced crosslinked polyester formulations were prepared and investigated. Solid glass microspheres, with and without coupling agent, were used as reinforcement. All samples were exposed to various cyclic hygrothermal fatigue patterns, differing from one another by the temperature and duration of a cycle. Dynamic mechanical measurements were used to evaluate the effect of hygrothermal fatigue on the mechanical properties of all formulations. Fatigue induced changes in the glass–matrix interphase led to the appearance of relaxation peaks (β′ and β″) in the temperature range between −50°C and −10°C. Electron microscopic evidence was obtained for the existence of nodular morphology on all fracture surfaces. The character of nodular morphology was described in terms of an inhomogeneous thermoset network.