Iron(II) thiochelates, especially iron(II) 2,3-bis(sulfanyl)propane-1-sulfonate (Fe(II)(DMPS)(2)), have received particular attention as nitric oxide (NO) absorbents because they maintain high NO removal efficiency even in the presence of O-2 in flue gases. Herein, we explored the O-2-inducing oxidation kinetics of Fe(II) and thiol groups (R-SH) in Fe(II)(DMPS)(2) and analyzed the resultant products to understand the resistive behavior toward O-2. Quantitative analysis revealed that 4 mM of Fe(II) in 10 mM Fe(DMPS)(2) solution was maintained without further oxidation even after 12 h under 5% O-2, but instead thiol group concentration continued to decrease. In comparison to the commonly used Fe(II)(EDTA) absorbent, in which all the 10 mM Fe(II) was fully oxidized within 40 min reaction by 5% O-2, this distinctive oxidative behavior of iron(II) thiochelate was attributed to the self-sacrificing thiol-containing ligands. This phenomenon was verified by the rapid and almost complete redox reaction between the oxidized Fe(III) and thiol groups in DMPS. Fourier transform infrared spectra indicated that the oxidation products of thiols were disulfide (S-S) and some sulfonates (-SO3-). Finally, based on all the experimental data, possible oxidation pathways of Fe(II)(DMPS)(2) by O-2 were proposed. This study provides both fundamental and applied insights of harnessing iron(II) thiochelate as a competitive NO absorbent for mitigating air pollution.