The photoelectron spectrum of the o-benzyne negative ion displays transitions to both the X1A1 and ã3B2 states of the neutral molecule. Results yield adiabatic electron affinities of 0.560 (10) and 0.551 (10) eV for C6H4 and C6D4, respectively. These values are consistent with extrapolations from studies of larger strained cycloalkynes and imply that the lowest unoccupied MO of o-benzyne is primarily an antibonding acetylenic orbital. The o-benzyne singlet-triplet splitting is determined to be 37.7 (6) kcal/mol. Several new vibrational frequencies for the neutral and anionic species are also reported.