Abstract A numerical and experimental study was conducted, with the purpose of inferring the influence of the CO2 concentration ( x CO 2 ) for different equivalence ratios ( ϕ ) on CH4/CO2/air (biogas) flames chemiluminescence. A thorough analysis on the signals of OH∗, CH∗, C 2 ∗ and CO 2 ∗ was performed. Typical biogas compositions were tested under laminar atmospheric flame conditions, within the unburned equivalence ratio of 0.9 and 1.14 with CO2 concentrations up to 40% in the blend. Experimental measurements of chemiluminescence were done using spectroscopy in the UV–visible region of the spectra. Simulations were performed with the GRI-Mech 3.0 mechanism without accounting for the nitrogen chemistry, extended with a chemiluminescence kinetics of OH∗, CH∗, C 2 ∗ and CO 2 ∗ , in a burner-stabilized frame in CANTERA. Experimental measurements and numerical simulations are compared and generally are in good agreement. It was verified that CO2 dilution leads to a regular decrease in the emission intensities of OH∗, CH∗, C 2 ∗ and CO 2 ∗ . Relations between chemiluminescence intensity ratios and x CO 2 and ϕ were found. It was shown that OH∗/ CO 2 ∗ and C 2 ∗ / CO 2 ∗ have the potential predict x CO 2 in CH4/CO2/air flames. Moreover, it was verified that OH∗/CH∗, OH∗/ C 2 ∗ and CH∗/ C 2 ∗ are well suited to infer ϕ for the blends tested. It was verified that x CO 2 does not cause relevant changes in the chemiluminescence ratios when inferring ϕ .