A photoelectron spectrum is reported for gas-phase SO4- formed in an electron-impact ionized free jet of SO2(1%)/O-2(10%)/Ar. The vertical detachment energy (VDE) of the SO4- 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 SO4- extracted from an Na2S2O8 solution. With the aid of MP2/6-311+G(2df) calculations, the newly formed species is identified as a peroxy form of SO4-, which is well described as OOSO2-(C-1, (2)A) having an S-O linkage between the O-2 and SO2 units. The electronic properties of OOSO2- are characterized by the singly occupied molecular orbital (SOMO) constructed by an out-of-phase superposition of the 1a(2) orbital of SO2 and the half-filled 2p pi(g)* of O-2(-). On the basis of these findings, a possible formation mechanism of OOSO2- is discussed in terms of a "solvent-mediated" addition reaction, where O-2(-) stabilized by solvation attacks SO2 while retaining the 2p pi(g)* excess electron.