The free radical copolymerization of ethene (ET) with acrylonitrile (AN) in the presence of carboxylic acids has been studied. Carboxylic acids and trifluoroacetic acid (TFA) in particular were found to be useful complexing agents for acrylonitrile with regard to the alternating copolymerization with ethene. The complexation via hydrogen bonding could be observed by IR analysis of the stretching frequency of the nitrile group in the monomer. The complex formation resulted in a decrease of the copolymerization parameter r(1) of the complexed and the uncomplexed comonomer acrylonitrile, which was found to decrease with increasing TFA/AN ratio. The degree of alternation could be increased by increasing the TFA/AN or the ET/AN ratio in the initial polymerization mixture. Almost strictly alternating copolymers mere obtained with TFA as solvent. The copolymers showed high crystallinity and melting temperatures similar to the strictly alternating copolymer, which has been synthesized in the presence of EtAlCl(2) as a Lewis acid.