Flash vacuum thermolysis (FVT) is a particular method that allows the synthesis of stable compounds or the generation of short-lived species. Its coupling with spectroscopic characterisation provides very useful tools for mechanistic investigations. One of the most efficient and especially well suited techniques for this purpose is ultraviolet–photoelectron spectroscopy (UV-PES), which in tandem with FVT provides the ionisation energies of in situ formed molecules in the gas phase. The experimental data thus obtained in real-time are supported by quantum chemical calculations for consistency of the assignments of PES spectra and constitute fundamental information about electronic structure and bonding. The FVT/UV-PES technique has been known for more than 40 years, but one advantage in the present age is the greater confidence in electronic structure computations for predicting ionisation energies, thus providing the necessary tool for unambiguous interpretation of experimental data. This mini-review aims to give some representative, original examples, chosen from a French–Polish collaboration that illustrates the efficiency and wide applicability of the FVT/UV-PES tandem methodology. The selected examples on the FVT of thione and imine derivatives will be presented.