Abstract The optical emission spectra of high pressure CO₂ microwave plasmas are usually dominated by the C₂ Swan bands. In this paper, the use of the C₂ Swan bands for estimating the gas temperature in CO₂ microwave plasmas is assessed. State by state fitting is employed to check the correctness of assuming a Boltzmann distribution for the rotational and vibrational distribution functions and, within statistical and systematic uncertainties, the C₂ Swan band can be fitted accurately with a single temperature for rotational and vibrational levels. The processes leading to the production of the C₂ molecule and particularly its d 3 Π g state are briefly reviewed as well as collisional relaxation times of the latter. It is concluded that its rotational temperature can be associated to the gas temperature of the CO₂ microwave plasma and the results are moreover cross-checked by adding a small amount of N₂ in the discharge and measuring the CN violet band system. The 2.45 GHz plasma source is analyzed in the pressure range 180–925 mbar, for input microwave powers ranging from 0.9 to 3 kW and with gas flow rates of 5–100 l min⁻¹. An intense C₂ Swan bands emission spectrum can be measured only when the plasma is operated in contracted regime. A unique temperature of about 6000 ± 500 K is obtained for all investigated conditions. A spectroscopic database is constructed using the recent compilation and calculations by Brooke et al (2013 J. Quant. Spectrosc. Radiat. Transfer 124 11–20) of the line strengths and molecular constants for the C₂ (d 3 Π g −a 3 Π u ) Swan bands system and made available as supplementary material in a format compatible with the open source MassiveOES software.