Abstract Ab initio molecular orbital calculations have been used to obtain the heat of formation of the acetyl cation. In one set of calculations, the reverse activation barrier for the production of acetyl cation from acetaldehyde has been shown to be significantly different zero and the value obtained (9.8 kJ mol −1 at 298 K) has been used to correct the Δ H∘ f298 (CH 3CO +) value derived from appearance energy measurements. In a second set of calculations, Δ H° f298 (CH 3CO +) has been obtained from the calculated heats of a number of reactions involving the acetyl cation together with experimental heats of formation for the species involved. The best theoretical estimate for Δ H° f298 (CH 3CO +), obtained as a mean of results from the two approaches, is 658 kJ mol −1. The best theoretical estimate for Δ H° f0(CH 3CO +), obtained in a similar manner, is 665 kJ mol −1.