A photoelectron spectrum is reported for gas-phase SO(4)(-) formed in an electron-impact ionized free jet of SO(2)(1%)/O(2)(10%)/Ar. The vertical detachment energy (VDE) of the SO(4)(-) species is determined to be 3.78 +/- 0.02 eV, which is significantly smaller than the VDE value reported for the sulfur-centered form of SO(4)(-) extracted from an Na(2)S(2)O(8) solution. With the aid of MP2/6-311+G(2df) calculations, the newly formed species is identified as a peroxy form of SO(4)(-), which is well described as OOSO(2)(-)(C(1), (2)A) having an S-O linkage between the O(2) and SO(2) units. The electronic properties of OOSO(2)(-) are characterized by the singly occupied molecular orbital (SOMO) constructed by an out-of-phase superposition of the 1a(2) orbital of SO(2) and the half-filled 2ppi(g)* of O(2)(-). On the basis of these findings, a possible formation mechanism of OOSO(2)(-) is discussed in terms of a "solvent-mediated" addition reaction, where O(2)(-) stabilized by solvation attacks SO(2) while retaining the 2ppi(g)* excess electron.