The present thesis is devoted to the study of the production mechanisms of H- and D- negative ions in a particular type of low temperature and low pressure microwave-driven plasma, an Electron Cyclotron Resonance (ECR, 2.45 GHz) plasma. On the basis of the collaboration between the High Voltage Laboratory at the University of Patras in Greece and the Laboratoire de Physique Subatomique et de Cosmologie in Grenoble, the two experimental setups utilized for this research are the Prometheus I reactor (HVL) and the SCHEME-II+ (LPSC). The former is primarily oriented towards the study of pure volume production of negative ions (through the well-established reaction of dissociative attachment) whereas in the latter, focus is set on assessing various plasma-facing materials (e.g. Quartz, Tantalum, Tungsten) for the surface-assisted volume production of these ions. Achieving a global picture of the produced plasma entails the use of multiple diagnostic techniques often complementary to one another, e.g. Langmuir probes, laser-induced photo-detachment, optical emission spectroscopy, vacuum ultraviolet and tunable diode laser absorption spectroscopy. Finally, both pure hydrogen and deuterium discharges are scrutinized, in addition to two regimes of plasma operation in each case, continuous and pulsed.