In this study, we explored the mechanistic features of CO₂ in the thermolysis of PVC to modify the pyrogenic products. The mechanistic roles of CO₂ in the thermolysis of PVC were varied with the pyrolytic temperature. For example, CO₂ participated in unknown reactions to form CO at temperatures higher than 600 °C, where CO₂ played a key role as an oxygen donor (new finding). This new thermal behavior induced by CO₂ could be a new way to shift carbon from pyrolytic oil to syngas (i.e., H₂ and CO) without using catalysts. This identified reaction that formed CO was noticeably enhanced in the presence of CaCO₃ by up to 12 times, which was proportional to the loading of CaCO₃. The stepwise thermal degradation mechanisms for PVC (i.e., dehydrogenation followed by dechlorination) provided favorable conditions for forming benzene derivatives and PAHs in pyrolytic oil. In the temperature range from 480 to 600 °C, CO₂ played a key role in restricting the formation of benzene derivatives, which subsequently lowered the formation of PAHs by blocking the pathway for the gas-phase addition reactions. At the current stage of this study, the operational parameters such as the exact amount of CO₂, space velocity for CO₂, and other parameters for the thermolysis of PVC were fully optimized. However, all the experimental findings marginally described the possible utilization of CO₂ for modifying pyrogenic products, which can be applied in various fields such as air pollutant controls (APCs) and energy applications. As such, further studies need to be conducted in the near future. Copyright © 2018 Royal Society of Chemistry.