The dissociative photoionization onsets for the production of CH3CO+ + CH3. from acetone and CH3CO+ + CH3CO. from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of "hot" electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of 1 kJ/mol. The experimental onset for CH3. loss from CH3COCH3 was measured to be 10.563 +/- 0.010 eV and the onset for CH3CO. loss from CH3COCOCH3 was found to be 10.090 +/- 0.006 eV. A 298 K heat of formation of the CH3CO+ of 659.4 +/- 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH3CO. radical of -9.8 +/- 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH3CO+. The acetone and butanedione ionization energies were measured to be 9.708 +/- 0.004 and 9.21 +/- 0.05 eV, respectively.