A dissociation mechanism of the S-S bond in the alpha,alpha'-dinaphthyl disulfide radical anion (NpSSNp center dot-) in organic solution was investigated on the basis of transient absorption measurements and DFT calculations. NpSSNp center dot- generated during gamma-radiolysis of NpSSNp in MTHF at 77 K showed the absorption band at 430 nm, which shifted to 560 nm with an increase of the ambient temperature up to room temperature. With the aid of DFT calculations at the B3LYP/6-31G(d) level, the shift of the absorption band was interpreted in terms of molecular conformational changes of NpSSNp center dot- due to the elongation of the S-S bond. It was observed that NpSSNp center dot- dissociates into naphthylthiyl radical and thionaphtholate anion in organic solution with a first-order rate constant in the magnitude of 106 s(-1). From Arrhenius plots of the decay rate constants of NpSSNp center dot- in a temperature range of 160-293 K, an activation energy for the S-S bond cleavage in NpSSNp center dot- in solution was determined along with a frequency factor. Based on the state energies of NpSSNp center dot- calculated at the B3LYP/6-31G(d) level, a Morse-like energy potential for the S-S bond cleavage of NpSSNp center dot- is depicted as a function of the S-S bond distance.