Thermolysis of methyl methanethiosulfinate and methyl tert-butyl sulfoxide has been studied by photoelectron (PE) spectroscopy. The electronic structure of methanesulfenic acid (1) generated from both compounds has been determined, and the thermal stability of 1 was checked. 1 appears rather stable in the gas phase, giving rise to thioformaldehyde and water at high temperature. Thermolysis of vinyl tert-butyl sulfoxide gives rise to ethanethial S-oxide (6). At the thermolysis onset, sulfine 6 is observed in a mixture with a compound identified as ethenesulfenic acid (4). These results imply either an easy isomerization of 4 to 6, in agreement with previous theoretical evaluations, or an alternative thermolysis pathway of the starting sulfoxide, directly leading to sulfine 6. The obtained PE spectra complement previous microwave and/or mass spectrometry data and provide a further insight in the electronic structure and thermal stability of sulfenic acids 1 and 4. The experimental ionization potentials are compared throughout this study with ab-initio calculated vertical ionization potentials either within Koopmans’ approximation or by difference between the ionic and ground state energies.