Summary: The evolution of a thermal runaway event is studied from the time a self- sustained temperature growth first sets in to the time deflagration flames begin to emerge. Proper modeling of the effect of conduction on the distribution of temperature growth reveals that it enhances the overall rate of release of chemical energy. It is shown that this contributes to the likelihood of substantial pressure increases being produced at some stage, and a criterion is identified for this to happen. In the absence of such pressure effects, the results are valid over a wide range of degrees of supercriticality, from marginal cases to cases in which conductive heat losses start off being very small indeed.