Flue gas recirculation (FGR) is an effective technology both to control NOx emissions during combustion processes and to control temperature and make-up for the volume of the missing N-2 during oxy-fuel combustion processes. In this article, a study of the individual role of the main products that are expected to form part of the recycled flue gas (CO, H-2, H2O, and CO2) on soot and gas products formed during the thermal decomposition of ethylene-additive gas mixtures in the 975-1475 K temperature range is reported. Experimental results obtained in a quartz flow reactor are examined with the main objective of assessing the effectiveness of each gas additive in suppressing or boosting soot formation. Additionally, experimental data have been interpreted in terms of a literature detailed gas-phase kinetic model to analyze the evolution of gas products and get a better understanding of the gas-phase processes involving the thermal decomposition of the ethylene-additive gas mixtures, although soot formation reactions are not included in such mechanism.