The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X~ 2B 2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3 ? and CU4 loss leads to new values of 78 ± 2 kJ mol-1 and 75 ± 2 kJ mol-1, respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 ± 1 kJ mol-1 derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a ?fH298 for CH3CO+ of 655 ± 3 kJ mol-1, which is 4 kJ mol-1 lower than that reported by Fogleman et al. The present ?fW298 for CH3CO+ can be combined with the ? fH298 for CH2CO (-47.5 ± 1.6 kJ mol -1) and H+ (1530 kJ mol-1) to yield a 298 K proton affinity for ketene of 828 ± 4 kJ mol-1, in good agreement with the value (825 kJ mol-1) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a ?fH298 (CH2CO+•) of 873 ± 3 kJ mol-1. © 2006 American Chemical Society.