Fundamental characteristics of the reaction-zone structure for CH4-CO2 jets, mimicking a fuel of high CO2 contents such as biogas, burning in a heated coflow on a piloted jet burner have been investigated. Jet flames under diffusion and MILD combustion conditions were studied using planar laser-induced fluorescence (PLIF) of O-atom radicals and CO for different CO2 contents of the fuel jet. Numerical simulations using detailed chemical kinetics were performed to gain further insights into the problem. The impact of fuel jet CO2 contents and jet velocity have been investigated. The main results include (a) quantification of O-atom radical and CO concentration from PLIF measurements, (b) numerical simulations of transition from conventional diffusion flame to MILD combustion, and (c) investigation of criteria for MILD combustion, including (i) peak flame temperature or the difference between peak flame temperature and cross-over temperature, (ii) peak CO and NO concentrations, and (iii) flame chemiluminescence.Numerical results show good agreement with experimental data for trends in CO and O-atom concentrations, shapes of spatial profiles, and quantitative CO concentrations. The PLIF and LES results show that in MILD combustion, concentrations of NO and CO are significantly lower than those in conventional flames. MILD combustion can be achieved in the present flames when the CO2 contents of the jet is 60 % or higher, when the difference between the peak flame temperature and the cross-over temperature is <550 K.