• A Co(III) complex with six nitrite ligands outperformed free Co 2+ in PMS activation. • Complexation with nitrite facilitated Co(II)/Co(III) cycling. • Radical-induced oxidation occurred in parallel with the Co(IV)-induced oxidation. • Complexation of Co(II) with nitrites resulted in an improved PMS activation capacity. • Nitrite addition enabled catalytic PMS activation by Co 2+ and Co 3 O 4 . In this study, hexanitrocobaltate (HN-Co(III)) was compared against Co 2+ with respect to the capability for peroxymonosulfate (PMS) activation. The role of NO 2 − ligands in enhancing Co(II)/Co(III) cycling was confirmed based on the two-fold increase in the benzoic acid (BA) degradation efficiency of HN-Co(III)/PMS vs. Co 2+ /PMS and the negligible reduction in the activation performance of HN-Co(III) over repeated use. This was in line with accelerated BA degradation and catalytic activity recovery upon NO 2 − addition to Co 2+ /PMS. A negative shift in the anodic peak potential of the Co(II)/Co(III) couple and an increase in the anodic current over time imply that NO 2 − coordination promoted the capacity of Co(II) for PMS activation and facilitated the Co(III)-to-Co(II) conversion through hydrolysis. Along with sulfate radical as the primary oxidant, high-valent cobalt acted as the non-radical secondary oxidant based on the transformation of phenyl methyl sulfoxide to the corresponding sulfone in the HN-Co(III)/PMS mixtures.