Dissociative photoionization mass spectrometry has been used to measure appearance energies for the I-hydroxyethyl cation (CH3CH=OH+) formed from ethanol and 2-propanol. Molecular orbital calculations for these two unimolecular fragmentation reactions suggest that only methyl loss from ionized 2-propanol does not involve excess energy at the threshold. The experimental appearance energy of 10.31 +/- 0.01 eV for this latter process results in a 298 K heat of formation of 593.1 +/-1.2 kJ mol(-1) for CH3CH=OH+ and a corresponding absolute proton affinity for acetaldehyde of 770.9 +/- 1.3 0 mol(-1). This value is supported by both high-level ab initio calculations and a proposed upward revision of the absolute isobutene proton affinity to 803.3 +/-0.9 kJ mol(-1). A 298 K heat of formation of 52.2 +/- 1.9 kJ mol(-1) is derived for the tert-butyl radical.