Vibrational spectra of C6H6+ and C6D6+ in the ground electronic state have been measured by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four wave mixing in Kr gas. The ionization energies of C6H6 and C6D6 determined by one-photon MATI, 74551+/-5 and 74579+/-5 cm(-1), respectively, are similar to those reported previously. Vibrational spectra are much simpler than the previous zero kinetic energy photoelectron and MATI spectra obtained by two-photon excitation. Almost complete vibrational assignments for the cations have been possible, which will be useful for future theoretical studies of the Jahn-Teller effect in these cations. Implication from the present one-photon spectra agrees with the previous suggestion that the geometry of benzene cation in the ground electronic state belongs to the D-6h symmetry. (C) 2003 American Institute of Physics.