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.