The pore size distribution (PSD) of porous carbons is most often derived from the analysis of standard N-2 and Ar adsorption isotherms measured at 77 and 87 K. From the two gases, Ar is recommended (IUPAC Technical Report 2015) as more reliable for the PSD analysis due to its minimal specific interactions with the surface polar groups. Such interactions may influence the adsorption of N-2 molecules due to its significant quadrupole moment. In practice, however, using liquid Ar as a cryogen for Ar adsorption measurements may be challenging because of its high cost and limited availability in various parts of the world. In this study, we propose using O-2 adsorption isotherms for the PSD characterization of porous carbons. The quadrupole moment of O-2 is less than one-third of the value reported for N-2, and thus its susceptibility to specific interactions with polar groups is much smaller than that of N-2. We demonstrate a quantitative agreement between the PSD results derived from the adsorption isotherms of O-2 and N-2 measured at 77 K, and Ar at 87 K on four representative carbon samples. The PSD calculations are performed using adsorption models based on the two-dimensional non-local density functional theory (2D-NLDFT). (C) 2019 Elsevier Inc. All rights reserved.