G2 theory, which is normally very reliable for the prediction of reaction thermochemistry, is shown to be quite unsuitable for the prediction of heats of combustion. This arises because of significant errors in G2 energies for O2 and CO2 that reinforce rather than cancel one another in combustion equations. The errors increase with size of system. For example, errors of 31 and 163 kJ mol-1 are found for the heats of combustion of methane and benzene, respectively.