The particle size distributions of polychlorinated and polybrominated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs and PBDD/Fs, respectively; together labeled PXDD/Fs) in ambient air in a suburban area in Beijing, China, were determined. The sums of the concentrations of the 17 2,3,7,8-PCDD/Fs and the sums of the concentrations of the 13 2,3,7,8-PBDD/Fs that were analyzed were 1499–2799 fg m–3 (95.4–175.4 fg I-TEQ m–3) and 1171–2424 fg m–3 (42.2–109.3 fg TEQ m–3), respectively. The PXDD/Fs were mainly (~90%) in the particulate phase. Significant linear correlations were found between the gas/particle partition coefficients (Kp) and subcooled liquid vapor pressures (PL0) of the PXDD/Fs. The regression coefficients indicated that the PCDD/Fs were mainly adsorbed to the particles and that the PBDD/Fs were mainly absorbed by the particles. The concentrations of the PXDD/Fs increased as the particle size decreased. The highest PXDD/F concentrations were found in the dae < 1.0 µm particles and more than 80% of the PXDD/Fs were found to be in the dae < 2.5 µm particles. Similar regression coefficients were found for the KP against PL0 for the different particle size fractions in the air. The PXDD/F distribution profiles in particles of different sizes were also studied. The lower chlorinated PCDD/Fs were found at higher concentrations in the coarser particles, and the higher chlorinated PCDD/Fs were mainly found in the finer particles. Polybrominated dibenzofurans, particularly the higher brominated dibenzofurans, were the dominant PBDD/F congeners. The contributions of the higher brominated dibenzofurans to the total PBDD/F concentrations decreased as the particle size increased, but that was not the case for the polybrominated dibenzo-p-dioxins.