The work describes in this thesis is a study of unstable species in the gas phase using photoelectron spectroscopy. The overall aim of this work was to study the photoionization behaviour of reactive specie using photoelectron spectroscopy with Synchrotron Radiation. UV photoelectron spectroscopy studies of the N and S atoms and the OH, OD, SH, NO and CF radicals 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. For the first time Rydberg series converging to the fourth ionization limit of S atoms have been observed and assigned have been proposed. The relative photoionization cross section of the first PE band of CF radicals has been measured as a function of photon energy and Rydberg series converging to the second PE band of CF have been observed for the first time. 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.