Abstract: Unsaturated polyester compositions which can be readily cured without the formation of gas are disclosed. The compositions contain a carbonyl-containing compound of the formula ##EQU1## wherein R.sup.1 is hydrogen, branched or unbranched alkyl, cycloalkyl, hydroxycarboalkylene, hydroxycarbocycloalkylene or hydroxycarboalkenylene, or substituted or unsubstituted aryl, aralkyl or hydroxycarboarylene; R.sup.2 is hydrogen, hydroxyl, hydroxyperoxy, alkyloxy or aryloxy, or R.sup.1 and R.sup.2 can be combined to form a substituted or unsubstituted alkylenecarboxy, alkenylenecarboxy or arylenecarboxy group. Examples are maleic acid anhydride, tetrahydrophthalic acid anhydride, formic acid and n-butyraldehyde. The carbonyl-containing compound is preferably added in an amount of 0.01 to 10% by weight, calculated on the unsaturated polyester.