The first biomimetic water-soluble Fe-II-porphyrin nitroxyl complexes were obtained and characterized by UV-vis in protonated and deprotonated forms by reduction of previously isolated and characterized (FeTPPSNO center dot)-T-II. The pK(a) involved in the Fe-II-HNO reversible arrow Fe-II-NO- + H+ equilibrium was estimated to be around 9.7. The (FeTPPSHNO)-T-II complex spontaneously reoxidizes to the nitrosyl form following a first-order kinetic decay with a measured kinetic constant of k = 0.017 s(-1). Experiments show that the HNO adduct undergoes unimolecular homolytic cleavage of the H-NO bond. DFT calculations suggest a phlorin radical intermediate for this reaction. The deprotonated NO- complex resulted to be more stable, with a half-life of about 10 min.