We report the synthesis of neodymium hydroxide nanoparticles via a nonaqueous and surfactant-free sol−gel process and their subsequent thermal transformation into neodymium dioxide carbonate, which can be applied as a sensing layer for resistive-readout CO2 sensing. The sensors show an increase in resistance when exposed to CO2 in both dry and humid air in the operation temperature range of 250−400 °C, with a maximum sensor signal of 4 in humid air at 350 °C in 1000 ppm CO2. Another important feature of the sensor is the fact that exposure to water vapor leads to a pronounced decrease in resistance (opposite of the CO2 effect), which indicates different charge-transfer mechanisms. The CO2 gas-sensing mechanism was studied via the Operando approach, by performing direct-current (DC) resistance and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements simultaneously under operation conditions. This combination enables the correlation of any concentration changes of specific surfa...