The results of computations with detailed chemical kinetic schemes for the autoignition of stoichiometric H-2-CO-air and H-2-air mixtures at high pressure and high temperature are reported, with and without a single hot spot. Autoignition delay and excitation times first are computed in zero-dimensional, homogeneous mixture, simulations. Spherical hot spots of three different radii are then studied, for a range of temperature differences between the centre of the hot spot and the surrounding mixture. The effects of the resulting localised initial temperature gradients on the propagation modes of the ensuing reaction waves are examined, with particular regard to possible transitions to a developing detonation. Five modes of reaction front propagation are identified and demonstrated. One mode involves normal flame deflagration, the other four involve different types of hot spot autoignition. These modes depend upon the value of the initial hot spot temperature gradient normalised by the critical temperature gradient for a developing detonation. The latter is conveniently obtained from the homogeneous computations. Upper and lower limits of this normalised temperature gradient,; are observed for a developing detonation. The bounds for this also depend upon the ratio of the hot spot acoustic time to the heat release rate excitation time. A tentative first attempt is described to quantify the bounds for all the modes, in terms of the two dimensionless groups. (C) 2003 The Combustion Institute. All rights reserved.