Abstract: The present invention relates to unsaturated polyester resin or vinyl ester resin compositions that are essentially free of cobalt; that are curable with a peroxide component; and that, comprise a polymer containing reactive unsaturations, optionally a reactive diluent, and a dioxo component; which dioxo component consists of one or more dioxo-compounds having a calculated reduction energy (CRE), being the energy difference in kcal/mole between the lowest energy conformations of the radical anion and the corresponding neutral molecule as calculated by means of the Turbomole Version 5 program, in the range of from ?5 to ?30 kcal/mole. These resin compositions show a reduced gel-time drift tendency. The present invention further also relates to objects and structural parts prepared from such unsaturated polyester or vinyl ester resin compositions by curing with a peroxide. The present invention finally also relates to methods of peroxide curing of unsaturated polyester resin or vinyl ester resin compositions.