The changes in the soot-related radiation intensity between two different oxy-fuel flames and an air-fired flame were investigated in a 100 kW oxy-fuel test unit firing propane. The oxy-fuel test cases with 21 and 27 vol % O-2 in the recycled flue gas (RFG) were run with different amounts of dry RFG, which in principle only consisted of CO2 from combustion and some excess O-2. The stoichiometric oxygen-to-fuel ratio was kept at 1.15 in all cases. Total radiation intensity was measured with a narrow angle radiometer. Temperature and gas composition measurements served as inputs to computations of gas radiation. A comparison of the computed gas radiation with the measured total radiation intensity enabled estimation of the radiation related to soot. Clear differences were observed in the amount of soot formed in the two oxy-fuel flames (also compared to the air flame). In the oxy-fuel flame with 21 vol % 02 in the RFG, soot formation is almost completely suppressed, but when the total flow through the burner is reduced by about 20% (by volume) (i.e., from 21 to 27 vol % 02 in the RFG), the amount of soot present in the flame becomes significant. This change in soot volume fraction affects the radiation emitted from the flames; images of the flames qualitatively confirm these differences in the flame luminosity. Thus, carbon dioxide not only increases the gas radiation, but it can also drastically influence soot formation and the radiation originating from soot during oxy-fuel combustion.