High-level ab initio calculations, in the framework of the G2(MP2) theory, have been carried out on the different tautomers of thiomalonaldehyde (TMA). These calculations are compared with those obtained using density functional theory methods, namely B3LYP, with extended basis sets, In general the enethiol tautomers of TMA are 5-10 kcal/mol more stable than the corresponding enol analogues, with the only exception being the Z-enol (E1) and the Z-enethioi (T1) hydrogen-bunded species, which are the global minima of both series. At the G2(MP2) level both tautomers are nearly degenerate, the enethiol T1 being 0.2 kcal/mol more stable than the enol E1. Electron correlation effects stabilize preferentially the enol form, while the ZPE corrections go in the opposite direction, due essentially to the differences between S-H and O-H stretching frequencies. As a consequence, when the hydrogen atom involved in the intramolecular hydrogen bond (IHB) of both tautomers is replaced by deuterium, the stability order is reversed and El is predicted to be more stable than T1. An analysis of these IHBs in terms of the topological characteristics of the electron charge density and of the shifts of the S-H and O-H vibrational frequencies reveals that the HE in Ea is much stronger than in T1. The existence of this IHB results in an increase of the electron delocalization which enhances the stability of tautomer E1. At the G2(MP2) level two open-chain rotamers, namely T4 and T7, are predicted to be within an energy gap smaller than 0.5 kcal/mol with respect to the global minimum. The use of continuum and discrete-continuum models indicates that both open-chain enethiols and enols are significantly stabilized by solute-solvent interactions, and they should predominate in aqueous solution. B3LYP/6-311+G(3df,2p) relative stabilities are in excellent agreement with G2(MP2) values.