This thesis presents vapour phase photoelectron spectra of a number of atoms and small molecules produced by high temperature vapourisation. A major feature of the work has been the development of a unique multidetector photoelectron spectrometer and the experimental details of this instrument are described in Chapter Two. Comparison of spectra obtained using both a single detector and the multidetector spectrometer has revealed significant improvement in sensitivity when using the multidetector instrument and this is very important at high temperatures. Theoretical methods, notably Hartree-Fock-Slater DV-Xα calculations, have been used to aid assignment of experimental spectra and these are described in Chapters Three and Four. In Chapter Four, the theoretical prediction of photoelectron band positions and vibrational structure has been attempted by calculation of ionisation energies, spectroscopic constants and Franck-Condon factors for the molecules CO, N_2, BF, AlF, TiO and VO. The success of these predictions has been investigated by comparison of computed spectra with the known experimental spectra and in the case of VO, this comparison was used to calculate an equilibrium bond length for the ground electronic state of VO^+. In Chapter Five, the Hel photoelectron spectrum of MgF_2 is reported and discussed in comparison with the known photoelectron spectrum of MgCl_2. The Hel photoelectron spectra of the transition metals Sc and V are presented in Chapter Six. This chapter also incorporates a comparison of the Hel σ4s : σ3d photoionisation cross-section ratios for Sc and V with those of the other first row transition metals. An investigation of the vapour phase products over a vanadium-oxygen system at different stoichiometries is described in Chapter Seven. In this study, the Hel photoelectron spectra of VO and V_4O_10 were observed. In Chapter Eight, photoelectron spectra are recorded for the vapour above heated mixtures of Al_(s) and Al_2O_3(s). Identification of the Al and Al_2O vapour species was aided by the use of a quadrupole mass spectrometer adapted in this work to sample the vapour phase species in the photoionisation region of the p.e.s. spectrometer. Finally, some incomplete Hel photoelectron spectroscopic studies of UO_2, BaO and NbO are presented in Chapter Nine, together with proposals for future developments and studies.