A simple model is presented which can be used to predict the detonation limits of gas mixtures. The model postulates that the energy required to establish a shock travelling at the detonation velocity (calculated from the Chapman-Jouguet theory) equals the chemical energy of the mixture at the detonation limit. This takes no account of the complex behaviour of near limit detonations, which show spin effects and varying velocity. In spite of this lack of realism the model is shown to predict limits in reasonable agreement with the measured values for a range of gas mixtures, including CH4—O2mixtures diluted with N2, or Ar and thirteen fuel-lean hydrocarbon—O2, mixtures. The predictions for fuel-rich hydrocarbon—O2, mixtures were less satisfactory, possibly because chemical equilibrium is too slow to occur in these detonations and H2—O2-diluent detonation limits are more accurately predicted by a criterion based on a fixed shock temperature.