The work presented in this thesis concerns the study of small reactive molecules by ultraviolet photoelectron spectroscopy. The molecules were produced by rapid atom-molecule and molecule-molecule reactions in the gas phase using several different inlet systems. The experimental techniques as well as the ab initio calculations used to assist the assignment and interpretation of the photoelectron spectra recorded are described. The HeI photoelectron spectrum of the CF radical, produced by the F + CH₃F reaction, is presented. The first and second bands of CF are recorded, where the first band is in good agreement with that previously published. The second band has an adiabatic ionisation energy of 13.94 ± 0.02 eV, and analysis of the vibrational structure gives w_c and r_e as 1614 ± 30cm⁻¹ and 1.213 ± 0.005 Ǻ respectively in the ionic state. The HeI photoelectron spectra, matrix isolation infrared spectra and thermal decomposition of 2-azidoethanol and 2-azidoethylacetate are presented. Ab initio calculations were performed in order to help interpret and assign the spectra recorded. Two mechanisms of decomposition are proposed, 2-azidoethylacetate decomposes via a concerted process through a cyclic transition state whereas 2-azidoethanol decomposes via a stepwise mechanism though an imine intermediate, which remains undetected. The HeI photoelectron spectra of molecules that are formed from the reactions of different atomic and molecular halogens with dimethylsulphide, dimethyldisulphide and diethylether are presented. The photoelectron spectra of the DMS:CI₂ complex is presented as well as suggested reaction mechanisms for all the reactions studied. Ab initio calculations have also been performed in order to help interpret and assign the spectra obtained.