The ThO+ cation is of interest as it is a useful prototype for experimental and theoretical studies of bonding in a simple actinide compound. Formally the ground state of ThO+ has the configuration Th3+(7s)O2−, where there is a single unpaired electron associated with a closed-shell Th4+-ion core. The first tier of excited states above the XΣ+2 ground state is expected to be 1Δ2, 1Π2, and 2Σ+2 derived from the Th3+(6d)O2− configuration. Spectroscopic observations of ThO+ using the pulsed field ionization–zero kinetic-energy photoelectron technique are reported here. Rotationally resolved spectra were recorded for the XΣ+2, 1Δ2, and 1Π2 states. Extensive vibrational progressions were observed. Surprisingly, it was found that ionization of ThO decreases the dissociation energy, while increasing the vibrational frequency and decreasing the bond length. Accurate values for the ionization energies of ThO [53253.8(2)cm−1] and Th [50868.71(8)cm−1] were determined as part of this investigation.