Prometheus I is a volume H- negative ion source, driven by a network of dipolar electron cyclotron resonance (ECR) sources (2.45 GHz)1–3. The vacuum-ultraviolet (VUV) emission spectrum of low-temperature hydrogen plasma is potentially related to molecular and atomic processes involved directly or indirectly in the production of negative ions4. In this work, VUV spectroscopy has been performed in above source Prometheus I. The acquired VUV spectra are correlated with the negative ion densities, as measured by means of laser photodetachment5, and the possible mechanisms of negative ions production are considered. The well-established H- formation process of dissociative attachment to vibrationally excited molecules6 is evaluated, while additional production paths (e.g. neutral resonant ionization7,8) are tested. The obtained results indicate that for the source Prometheus I, the dominant formation process is dissociative attachment.