X-ray photoelectron spectroscopy has been used to record the O_1s and S_2p ionization spectra of thioacetylacetone, 2-acetylcyclohexanethione, 2-thioacetylcyclohexanone, the S-methyl derivative of thioacetylacetone, and propyl 3-mercaptocrotonate in the gas phase. It is shown that both the enol and the enethiol tautomers of the β-thioxoketones can be detected, and the enol/enethiol ratios for thioacetylacetone, 2-acetylcyclohexanethione, and 2-thioacetylcyclohexanone were determined to be 61:39, 30:70, and 80:20, respectively, based on the intensities of the oxygen ionizations. The conclusions derived from the sulfur region support the above, although they are less clear due to sulfur spin-orbit splitting. The enol/enethiol ratios obtained in the gas phase by XPS are compared with data from other methods, showing good agreement between results obtained in the gas phase and in solution. The binding energy (BE) shifts of the O_1s and S_2p orbitals have been discussed and suggest that geometrical factors affect the strength of the intramolecular hydrogen bond.