The complex of iron and tetra-amido macrocyclic ligand (Fe-TAML), or Collin’s reagent, is a promising catalyst to carry out Fenton-like degradation of contaminants. Previously, the ability of Fe-TAML to activate sulfite and metabisulfite (S2O52-, MBS) other than hydrogen peroxide, was reported, which could lead to cheaper, safer, and almost equally efficient oxidation of contaminants. However, the actual species involved in the Fe-TAML/MBS is not yet clear. Experiments reported here included a combination of tests involving the presence of various scavengers, modified atmospheres (air vs. N2), as well as measurements with electron paramagnetic resonance. Specifically, the tests were designed to positively exclude the presence of various active species and identify the most likely one. Thus, we theorize the involvement of the peroxymonosulfate radical (SO5·-) in the presence of the Collin’s reagent and metabisulfite and provide indirect evidence of the activity of this species. Specifically, we demonstrated that this system triggers a Fenton-like process, able to induce the degradation of phenol. The exploitation of such a reactive species may be favourable, as it may selectively induce the degradation of target contaminants in the presence of excess interfering agents, starting from the cheap, safe, and easily handled metabisulfite, which is an interesting substitute for hydrogen peroxide.