A composition comprising at least two trimeric cyclic ketone peroxides: a trimeric cyclic methyl ethyl ketone peroxide (3MEK-cp) of formula (I) and at least one peroxide satisfying formula (II) wherein R1 through R3 are independently selected from alkyl and alkoxyalkyl groups, said groups having 2 to 5 carbon atoms, the total number of carbon plus oxygen atoms of R1−R2+R3 is in the range 7-15, and the molar ratio of 3MEK-cp to the total amount of peroxides satisfying formula (II) being in the range of from 10:90 to 80:20.