The present invention relates to a pre-accelerated resin composition comprising an unsaturated polyester resin and/or a vinyl ester resin, wherein the composition further comprises a. a copper salt, and b. a compound according to formula (1) in which R can be selected from H, optionally substituted C1-C20 alkyl, optionally substituted C6-C20 aryl, and wherein the copper is present in an amount of more than 0.1 mmol/kg resin.