2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide, as a redox mediator for batteries, for the activity estimation of antioxidants, and so on. However, there is no report on the reactivity of PTIO in the presence of redox-inactive metal ions. In this study, it is demonstrated that the addition of scandium triflate, Sc(OTf) (OTf = OSOCF), to an acetonitrile (MeCN) solution of PTIO resulted in an electron-transfer disproportionation to generate the corresponding cation (PTIO) and anion (PTIO), the latter of which is suggested to be stabilized by Sc to form [(PTIO)Sc]. The decay of the absorption band at 361 nm due to PTIO, monitored using a stopped-flow technique, obeyed second-order kinetics. The second-order rate constant for the disproportionation, thus determined, increased with increasing the Sc(OTf) concentration to reach a constant value. A drastic change in the cyclic voltammogram recorded for PTIO in deaerated MeCN containing 0.10 M BuNClO was also observed upon addition of Sc(OTf), suggesting that the large positive shift of the one-electron reduction potential of PTIO (equivalent to the one-electron oxidation potential of PTIO) in the presence of Sc(OTf) may result in the disproportionation. When HO was added to the PTIO-Sc(OTf) system in deaerated MeCN, PTIO was completely regenerated. It is suggested that the complex formation of Sc with HO may weaken the interaction between PTIO and Sc, leading to electron-transfer comproportionation to regenerate PTIO. The reversible disproportionation of PTIO was also confirmed by electron paramagnetic resonance (EPR) spectroscopy.