Two experimental studies determining the ionization potentials (IPs) of a nitrate free radical have recently been done. The first study performed using discharge flow-photoionization mass spectroscopy (PIMS) determined the adiabatic ionization potential at 12.57 eV. The second study using the HeI photoelectron spectroscopy (PES) observed five bands with the five lowest vertical ionization potentials of NO3 at 12.55, 13.18, 13.62, 14.05, and 15.54 eV. The second IP determined using PES (at 13.18 eV) has been questioned due to its absence in the spectrum obtained using the PIMS experiment. We use our theoretical method, the multiconfigurational spin tensor electron propagator method (MCSTEP), to help resolve the controversy over the ionization potential of NO3. We perform MCSTEP calculations on NO3 using the standard cc-pVDZ and cc-pVTZ basis sets at D-3h and C-2v geometries. Our results at both of these two symmetries support the experimental results obtained using PES. (C) 2000 American Institute of Physics. [S0021-9606(00)30610-9].