The free-radical terpolymerization of maleic anhydride (MSA), isobutyl vinyl ether (VIBE) and anethol (ANE) at 60 degrees C is described. These three monomers do not homopolymerize under the conditions used. Binary copolymers are only obtained from MSA and one of the two investigated donor monomers, whereas the binary copolymerization of VIBE with ANE is not possible. In all terpolymers from these three non-homopolymerizable monomers the MSA content amounts to about 50 mol-%; VIBE is about twice as reactive as ANF. The terpolymerization can formally also be described under the assumption that two binary complexes (MSA/ANE (I) and MSA/VIBE (II)) are copolymerized. For this case the reactivity ratios r(I) = 0,9 and r(II) = 1,7 are obtained.