Abstract: To explore the feasibility of using chlorine isotope abundance ratio to investigate the degradation of organochlorine pollutants in air, gas chromatography-high resolution time-of-flight mass spectrometry (GC-HR-TOF MS) method was carried out for calculation of chlorine isotope abundance ratio (³⁷Cl/³⁵Cl) based on measurements of atmospheric hexachlorobenzene (HCB) in both industrial (Tanggu and Hangu) and rural (Yuqiao) sites in Tianjin. Air samples were collected using passive air sampler with XAD-2 resin as absorbent. Interference free HCB peaks were obtained as indicated by accurate masses and good spectral match. The chlorine isotope abundance ratio (³⁷Cl/³⁵Cl) of HCB was derived from its isotopic peak profile. Since fragmentation may complicate the situation, molecular ions were preferred. Most of the mass peaks ought to be taken into calculation，however, inclusion of small fragment ions may deteriorate the precisions of the chlorine isotopic ratios finally calculated. The experimental data indicates that the precision (RSD) of ³⁷Cl/³⁵Cl ratio ranges 2‰-5‰. The chlorine isotope abundance ratio (³⁷Cl/³⁵Cl) of HCB in industrial Tanggu and Hangu sites was 0.319 3±0.001 6 and 0.320 7±0.000 8, respectively,whereas that in rural Yuqiao was 0.323 9±0.000 8; the former was statistically lower than the later, as expected theoretically, which was attributable to the degradation of HCB during the atmospheric transport from source region to receptor sites. The chlorine isotope abundance ratios (³⁷Cl/³⁵Cl) of HCB obtained in this work are comparable and in good agreement with those in previous studies. It is encouraging and promising to use GC-HR-TOF MS to determine the chlorine isotope abundance ratio (³⁷Cl/³⁵Cl) of organochlorine pollutants such as HCB, and, in turn, to investigate related environmental processes such as degradation. Further studies could investigate the effects of different ionization methods and conditions, concentration levels of target compounds on the isotopic abundance ratios and its precisions obtained. It could be extended to more chlorine-containing pollutants of interest in other environmental matrices.