The HONO radical has recently been observed by photoionization mass spectrometry in low-pressure hydrogen-oxygen flames doped with NO2. The photoionization efficiency (PIE) spectrum has been measured between 10.83 and 11.63 eV. A Franck-Condon simulation using calculated geometries and force constants of the cation and neutral, and including the effects of Duschinsky rotation, is presented to describe the PIE as a function of photon energy. The simulated PIE is used as a fitting function to estimate the adiabatic ionization potential from the experimental data. The apparent ionization threshold of (10.97 +/-0.03) eV is in excellent agreement with calculated values and is consistent with published bracketing determinations of the proton affinity of NO2. (C) 2004 Elsevier B.V. All rights reserved.