Thermionic materials are commonly used in high vacuum (< 10(-5) mbar). We measured thermionic emission of different doped metal-oxides cathodes -Zr doped Gd2O3 and Nb doped SrTiO3 (1 1 1) - in a CO2 surrounding, at T = 1300 degrees C and similar to 10(-4) mbar <= p <= 300 mbar. Experimental setup validation and reference data were provided by measurements using cathodes made of CeB6, a commercial thermionic material, and by using either helium or argon as the surrounding gas. The study's results could be used when examining enhancement of CO2 reduction to CO using vibration's excitation. The thermionic emission of all cathodes decreased gradually as the pressure increased, as expected, but the exposure to CO2 did not cause a deterioration of the Zr doped Gd2O3 cathodes. They retained a reasonable emission, of similar to 60% and similar to 40% of the current in vacuum, at a CO2 pressure of 50 mbar and 300 mbar, respectively, over 20 operation hours. The data suggest that in the presence of significant gas pressure in the cathodes surrounding, the electric dipole of CO2 molecules and the cathode's surface roughness may induce several mechanisms, which could affect the current emission.