Flame propagation in fuel-air mixtures that are undergoing slow combustion is directly relevant to combustion in internal combustion engines. In connection with knock there is the question of whether end-gas slow-combustion reactions can lead to flame speed promotion. In the present work various fuel-air mixtures were reacted in a burner tube of variable length under stationary flow conditions. The reacting gases, upon emerging from the nozzle at the top of the burner, were either analyzed by gas chromatography or ignited and their flame speed measured. Fuels investigated were n-heptane at 250–270°C, diisopropyl ether at 190–270°C, and various diisopropyl ether-diethyl ether mixtures at 195°C. The n-heptane and the ether mixtures produced cool flames. The results show that, in the absence of or before the cool-flame stage, the hot-flame speed is only slightly affected by the extent of prereaction. In the post-cool-flame gases no hot flame could be stabilized because of blowoff. Upon coalescence of cool and hot flames, flame speed increases of up to 80 per cent were observed. The mechanism of this flame speed promotion is discussed as well as the implication with respect to the mechanism of engine knock.