Abstract: The invention relates to a process for the controlled hardening of unsaturated polyester resins at temperatures of -5.degree. C. to +150.degree. C. using an organic peroxide initiator. According to the process of the invention an organic dyestuff of the general formula (I), ##STR1## wherein R.sup.1 stands for a group of the general formula (II), ##STR2## and in this formula R.sup.5 is a C.sub.1-4 alkyl group and n is an integer from 1 to 3,R.sup.2 and R.sup.3 each represent a C.sub.1-4 alkyl group,R.sup.4 stands for hydrogen, orR.sup.1 and R.sup.4 form together a group of the general formula (III), ##STR3## and in this formula R.sup.6, R.sup.7 and R.sup.8 each represent hydrogen or a C.sub.1-4 alkyl group and Z stands for oxygen or sulfur atom or a -NH- group, andX stands for chlorine or bromine, is admixed with the polyester resin and/or the organic peroxide in an amount of 0.001 to 0.05% by weight, calculated for the weight of the polyester resin.