Ab initio (HF and MP2) and density functional (B3LYP) calculations with different basis sets (6-31G*, 6-311 + G* * 6-311 + G(2df,p)) on the various conformers of (Z)-3-formyl-acrylic acid and the corresponding ring-closed tautomer 5-hydroxy-furan-2(5H)-one are performed. At all levels of theory the ring tautomer is predicted to be the most stable one. Inclusion of bulk solvent effects (H2O) by the self-consistent isodensity surface polarized continuum model (SCIPCM) further shifts the ring-chain tautomeric equilibria towards the furanone form. Among the open chain tautomers, the s-cis-s-trans conformation is the most stable.