Oxygen plays an important role during the thermal treatment of soil, contaminated with polychlorinated biphenyls (PCBs), due to the potential oxidation of PCBs to form polychlorinated dibenzofurans (PCDFs). The effect of oxygen content (0, 5, 21 and 100%) in carrier gases on PCBs and PCDD/Fs was studied both in soil and gas after thermal desorption of PCBs contaminated soil at 500 °C. All 209 congeners of PCBs and 136 congeners of PCDD/Fs (P = 4 to 8) were analysed. Oxygen content showed little effect on PCB removal and destruction. Under different carrier gases, the removal efficiency and the destruction efficiency for PCBs attained 93.8-95.5 and 83.0-85.0 %, respectively. The levels of PCDD/Fs in soil and gas were correlated positively with oxygen content. Compared with PCDDs, PCDFs in soil were not effectively removed under oxidative conditions because there was chemistry going on and PCBs were being converted to PCDFs. The total concentration of PCDFs in soil and gas was 2.6, 11.3, 15.6 and 17.5 times of the initial PCDFs concentration (21.9 ng/g) in raw soil with increasing oxygen content. Thus, substantial amounts of PCDFs were generated in the presence of oxygen during the treatment of contaminated soil.