The invention relates to an initiator system comprising (a) an iodonium salt, (b) a light sensitizer and (c) an electron donor compound comprising a biphenylene structure, the biphenylene structure comprising at least one but not more than about 4 alkyl groups. The invention also relates to a hardenable composition comprising such an initiator system and the use thereof, as well as to a process for producing the substituted biphenylene compound.