A spectroscopic method using the molecular spectrum of C2 to determine temperature in diamond chemical vapour deposition processes running at atmospheric pressure has been studied. Under atmospheric pressure conditions, the argon interferences cannot be ignored without inducing important errors. The precise simulation of emission spectra permits a rigorous line selection and background correction. Systematic comparison has been done with the values
of temperature obtained using iron spectrum. The resulting method is fast and easy to implement. The temperatures measured from iron and C2 spectra are close, suggesting that important difference between excitation and rotational temperatures sometimes quoted in literature could
be due, at least partly, to spectroscopic mistakes.