A study of the reactivity of soot produced from ethylene pyrolysis at different temperatures and CO2 atmospheres toward O2 and CO2 has been carried out using a thermogravimetric analyzer. The purpose was to quantify how soot reactivity is affected by the gas environment and temperature history of the carbon, as well as to compare the soot reactivity toward O2 and CO2. Soot samples were either oxidized in 5% O2 or gasified in 10, 50, and 90% CO2 atmospheres, during non-isothermal runs at 10 K/min. Soot oxidation was observed at temperatures of 400–500 K lower than soot gasification, showing higher reactivity toward oxygen than CO2. Independent of the environment history of the soot samples, the soot samples formed at lower temperatures have higher reactivity toward both O2 and CO2 than the soot samples obtained at higher temperatures. The presence of CO2 during the formation of the soot only affected the soot reactivity at the highest formation temperature (1475 K) and CO2 concentration (78.5%). Under these conditions, the soot reactivity was observed to increase by a factor of about 2.6 compared to soot formed in N2 at the same temperature. We attribute the increased reactivity to a higher micropore surface area, facilitated by the gasification reaction at a high temperature. The intrinsic kinetics for oxidation and gasification of soot were obtained by applying the volumetric reaction model.