The influence of manganese modification on the spectroscopic features of manganese-doped CeO 2 systems synthesized by the microwave-assisted hydrothermal route and their correlation with the presence of O defective structures were verified, focusing on their interaction with poisonous atmospheres. Raman and electron paramagnetic resonance studies confirmed the presence of defective clusters formed by dipoles and/or quadrupoles. The number of paramagnetic species was found to be inversely proportional to the doping concentration, resulting in an increase in the Mn 2+ signal, likely due to the reduction of Mn 3+ species after the interaction with CO. X-ray photoelectron spectroscopy data showed the pure system with 33% of its cerium species in the Ce 3+ configuration, with an abrupt decrease to 19%, after the first modification with Mn, suggesting that 14% of the Ce 3+ species are donating one electron to the Mn 2+ ions, thus becoming nonparamagnetic Ce 4+ species. On the contrary, 58% of the manganese species remain in the Mn 2+ configuration with five unpaired electrons, corroborating the paramagnetic feature of the samples seen in the electron paramagnetic resonance study.