To investigate the origin of the p-n transition observed in graphene-based materials for chemical sensors, we developed sensing layers made of chemically reduced graphene oxide (rGO), prepared from graphene oxide (GO) using citrate as a reducing agent. We focused mainly on the thermally induced p-n transition when the sensing layers are exposed to H2S at a specific temperature. The rGO sensing layers are prepared through drop casting on interdigitated electrodes. Scanning electron and transmission electron microscopes allow the characterization of the rGO structure, while FTIR, X-ray photoelectron spectroscopy, and electrical characterization have revealed the reduction efficiency. The p-n transition occurs between 80 °C and 90 °C, whatever the resistance range and the exposition history to H2S. The temperature-dependent transition, which starts around 80–90 °C, is reversible if the temperature is kept below 80 °C, i.e., the rGO remains p-type material. When experiments are performed at a temperature above 90 °C, the transition from p-type to n-type takes place, and the n-type persists, showing an irreversible transition. The transition starts gradually at 80 °C during lower concentration exposure and ends at high concentration. Finally, the results show that temperature and H2S contribute to the inversion of the initial charge carriers.