Electron affinity (EA) is an important molecular property relevant to the electronic structure, chemical reactivity, and stability of a molecule. A detailed understanding of the electronic structures and EAs of benzoquinone (BQ) molecules can help rationalize their critical roles in a wide range of applications, from biological: photosynthesis to energy, conversion;processes. In this Article, We report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers-o-, m-, and p-BQ-employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The PES spectra of the three BQ(O-) radical anions were taken at several photon energies under low-temperature conditions. Similar spectral patterns were observed for both o- and p-BQ(O-), each revealing a broad ground-state feature and a large band gap followed by well-resolved excited states peaks. The EAs of o- and p-BQ were determined to be 1.90 and 1.85 eV with singlet-triplet band gaps of 1.68 and 2.32 eV, respectively. In contrast, the spectrum of m.BQ(O-) is distinctly different from its two congeners with no dear band gap and a much higher EA (2.89 eV). Accompanied theoretical study confirms the experimental EAs and band gaps. The calculations further unravel a triplet ground state for m-BQ in contrast to the singlet ground states for both o- and p-BQ, The diradical nature of M-BQ which is consistent with its non-Kekule structure, is primarily, responsible for the observed high EA and helps explain its nonexistence in bulk materials.