The present invention relates to a curable composition containing a) a material which can be polymerised by free radicals or cations, mixtures of these materials or a material which can be polymerised by free radicals and cations, b) at least one iron compound of the formula I ##EQU1## in which a is 1 or 2 and q is 1, 2 or 3, L is a divalent to heptavalent metal or non-metal, Q is a halogen atom,