AbstractThe water absorption at 50°C and dynamic and isothermal thermogravimetric properties were studied for a series of resins made from the same maleic/phthalic anhydride‐based unsaturated polyester cross‐linked with each of styrene, 4‐methyl styrene, 4‐ethyl styrene, 4‐n‐butyl styrene, 4‐isopropyl styrene, tertiary butyl styrene, 4‐chlorostyrene and 3,4‐dichlorostyrene. Chloro substituents and most alkyl groups give rise to a loss of water resistence. However, the tertiary butyl group leads to a decrease in both water absorption and water damage. The thermal stability of the resin is decreased by chloro substituents and increased by n‐alkyl groups, and the results suggest that the main degradation step involves styrene residues, but that the initial decompos