Spontaneous reactions of 1-(2,2-dimethyl-1,3-dioxane-4,6-dione-5-ylidene)-4-(dicyanomethylene)-2,5-eyclohexadiene (QM 1) with p-methoxystyrene (MeOSt) were investigated in chloroform at room temperature at different monomer feed ratios. Reaction products obtained as hexane-insoluble and hexane-soluble fractions were characterized by elemental analysis and IR, H-1 NMR, and C-13 NMR spectroscopies. The hexane-insoluble products were to be copolymers of 7-(4-methoxyphenyl)-8,8-dicyanoquinodimethane (QM 2) with small amounts (3-6 mol %) of the QM 1, and the reaction product in the hexane-soluble fractions was to be the one-to-one adduct (Cycloadduct) of methylene Meldrum's acid (MM) with MeOSt. It was found that in the spontaneous reaction of the QM 1 with MeOSt a side reaction involving a metathesis process took place to form new two products, QM 2 and MM, and the former polymerized and the latter underwent the cycloaddition reaction with MeOSt. The formation of the Cycloadduct was also confirmed by a model reaction using 1-[(6-hydroxy-2,2-dimethyl-4-oxo-4H-1,3-dioxin-5-yl)-methyl]pyridinium hydroxide as a precursor of the MM and MeOSt. To identify the reactive intermediate, the spontaneous reactions of the QM I with MeOSt were carried out in the presence of 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) and methanol. No products trapped by TEMPO were obtained, but in the case of methanol the one-to-one-to-one adduct (Adduct) of the QM 1, MeOSt, and methanol was isolated. It was concluded, therefore, that the spontaneous reaction of the QM 1 with MeOSt might proceed via a zwitterionic tetramethylene intermediate.