The He I photoelectron spectra of thioacetylacetone, 2-acetylcyclohexanethione, 2-thioacetylcyclohexanone, and the S-methyl derivative of thioacetylacetone has been recorded and interpreted by comparison with PE data on related compounds. The interpretation has been supported by semiempirical molecular orbital calculations based on the CNDO/B, CNDO/S, and MNDO methods on the enol and enethiol forms of monothiomalondialdehyde. The three lowest ionization energies were determined as ns ͌π₃<π₂ for the enol form and as π₃ πn₀ <π₂ for the enethiol form. The influence of hydrogen bonding on the electronic structure is discussed. Attention is called to the possibility of distinguishing generally by means of PE spectroscopy between the enol and enethiol forms of a tautomeric equilibrium system under consideration.