In this study, five salts containing the same anion fac-[Fe^II(CO)₃I₃]⁻ were prepared through the reaction of cis-[Fe(CO)₄I₂] with organic amines whose protonated forms, i.e., (EtNH₃⁺), (PrNH₃⁺), (BuNH₃⁺), (PentNH₃⁺), and (Et₂NH₂⁺), become the cations of the salts 1, 2, 3, 4, and 5, respectively. These aminium salts, which are soluble in water and polar organic solvents such as DMSO and methanol, were characterized by various techniques including X-ray single-crystal diffraction. These salts underwent solvolysis in both water and DMSO to release CO, following first-order kinetics. In DMSO, the decomposition of these salts proceeded via a significantly simpler mechanism without the involvement of radicals than that in water, and the CO-release rate of these salts was almost linearly related to the hydrophobicity (the logarithm of the oil/water partition coefficient P) of the amines. The CO-release rate was substantially faster in water and in addition to undergoing solvolysis, the anions generated an iodine radical, I˙. Due to the complicated decomposition of these salts in water, the CO-release proceeded in a two-stage manner. The iodine radical was found to be at least partly responsible for the cytotoxicity of these salts in water; this indicates that the proposed anion has potential for applications in cancer therapy.