The radical terpolymerization of the donor-acceptor-donor monomer system, phenanthrene (P) - maleic anhydride (M) - trans-stilbene (S), was studied. These monomers are known to be nonhomopolymerizable. The terpolymerization was carried out in p-dioxane and/or toluene at 70-degrees-C in the presence of benzoyl peroxide used as the initiator. P and S were found to form charge transfer complexes (CTC) with M in p-dioxane at 35-degrees-C. The results obtained are discussed in terms of the free monomer and complex propagation models. It is shown that terpolymerization is carried out at a stage close to binary copolymerization of two complexomers. The reactivity ratio of P. . .M and S. . .M complexes was estimated by the Kelen-Tudos method. Absorbance ratios at 1770 cm-1 (upsilon(C = 0) of anhydride group), 764 cm-1 (delta(CH) in monosubstituted benzene of S), and 820 cm-1 (delta(CH) in disubstituted benzene of P) as a function of terpolymer composition was established. P-M-S terpolymers are shown to have high thermal stabilities.