The reactions leading to the propagation of a flame have been studied in shock-heated air containing small amounts of either (normal) octane or 1,1-dimethyl hexane (isooctane). Preflame reactions, associated with the production of excited hydroxyl radicals, have been shown to occur with both octanes. Such reactions would account for the observed acceleration of the flame with octane and its retardation with isooctane, provided that the products from octane were more reactive (as found here) and the products from isooctane less reactive (as found by Cullis for heptane) than the parent hydrocarbons. Isooctane, as expected, was more resistant to preflame reactions; nevertheless they were observed at temperatures as low as 1050 K. The temperature coefficients of the reactions governing the growth of chemiluminescent radiation are similar for the two octanes: 330 and 350 kJ mol-' for octane and isooctane respectively. The temperature coefficients for the reactions controlling the overall burning are lower, falling to about 190 kJ mol-'. The marked differences between the growth of pressure in what is essentially a premixed flame, and the growth of pressure in the combustion of droplets of (normal) hexadecane, are noted.