Spontaneous polymerization of 7,7-dicyanobenzoquinone methide (CQM) with p-methoxystyrene (MeOSt) was investigated. An alternating copolymer of CQM with MeOSt was obtained as the hexane-insoluble product, and a small amount of a 1:2 cycloadduct of CQM:MeOSt in addition to large amounts of unreacted CQM and MeOSt was obtained as the hexane-soluble product. To clarify an active intermediate in this reaction, spontaneous polymerizations of CQM with MeOSt were carried out in the presence of additives such as 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) and acetic acid and in three solvents with different polarity. The spontaneous reaction in the presence of the TEMPO did not afford any products trapped by TEMPO. On the other hand, the reaction in the presence of acetic acid gave a 1:1:1 adduct of CQM:MeOSt: acetic acid in a quantitative yield. It was concluded from these results that the spontaneous polymerization of CQM with MeOSt might proceed via a zwitterionic intermediate, which has gauche and trans forms.