The work described in this thesis is a study of unstable species in the gas phase using electron spectroscopy. The overall aim of this work was to study the photoionization behaviour of reactive species using photoelectron spectroscopy with Synchrotron Radiation. UV photoelectron spectroscopy studies of the O, N and S atoms and the OH, OD and SH radicals and a study of photodissociation of ethylene have been performed using Synchrotron Radiation. Constant ionic state (CIS) angular resolved experiments have been made. In these experiments, autoionizing resonances have been observed and assigned to members of series of Rydberg states converging towards higher ionic limits. Vibrational and rotational resolution has been obtained in OH and OD CIS spectra. For each reactive atom and molecule studied, measurement of angular distributions of photoelectrons allowed insight into the photoionization dynamics to be obtained. The photodissociation mechanisms of ethylene have been probed through the detection of visible fluorescence emitted by the fragments, using an optical device designed in Orsay. Based on the results obtained, mechanisms of photodissociation of ethylene at the photon energies used have been proposed.