CaCO3 was modified with monoallyl and monododecyl maleate. The modification caused a decrease of the dispersive component of the surface energy and of the energy-specific interactions of the filler. Using allyl esters of maleic, tetrahydrophthalic or succinic acids as coagents for peroxide crosslinking of ethylene-propylene rubber, vulcanizates were obtained which contained the covalent and ionic crosslinks in their structure. The unsaturated groups in the coagent alcohol rest were more reactive toward free radicals than those in the acid residue. The coagents were effective when the carboxyl group was neutralized, i.e. when the coagent is used in the form of a salt, or when it is deposited on the surface of a basic filler. The precipitated calcium carbonate modified with monoallyl maleate was most active as a filler. The modification of CaCO3 with monododecyl maleate, on the other hand, let to a decrease of its activity as compared to the unmodified form.