The tautomerism and spectral properties of 3-[3-(4-methoxycarbonylphenyl-acryloyl]tetrahydrofuran-2,4-dione (MCPATD) have been investigated by the methods of nonempirical and semiempirical quantum chemistry (nonempirical calculations by the Möller–Plesset theory of 2nd-order perturbations, calculations by the AM1 and PM3 semiempirical methods), as well as by IR and 1H NMR spectroscopy. It has been shown that the presence of an additional chain of conjugation in the side chain of MCPATD substantially changes its tautomeric composition and spectral properties as compared to 3-formyl- and 3-acetyltetrahydrofuran-2,4-diones. The frequencies and forms of normal vibrations calculated for each cis-enolic tautomer differ substantially within the region of vibrations of keto groups and double bonds, which makes it possible to identify the tautomers present in the mixture. It is found that in CHCl3 solutions MCPATD exists as an equilibrium mixture of its exoenolic forms. The possible mechanisms underlying the enol-enolic conversions of MCPATD are discussed.