Abstract Consistent data for the flammability limits of different hydrogen–carbon monoxide mixtures were determined experimentally over a wide range of compositions and initial temperatures up to 573 K (300°C) and at atmospheric pressure for upward flame propagation in a stainless-steel test tube apparatus. Experimental results also showed that the lean flammability limits of hydrogen–carbon monoxide mixtures obeyed Le Chatelier's Rule over the entire range of temperature considered when the corresponding individual limit values of the components were employed. However, the rich flammability limits deviated very significantly from the corresponding values calculated using this Rule, especially for mixtures containing small concentrations of hydrogen. The rich limits predicted by the Rule were narrower than those obtained experimentally. To investigate the existence of pre-ignition reactions at these levels of temperature and their effect on the flammability limits, the hydrogen–carbon monoxide–air mixtures were exposed to elevated temperatures over different periods of time varying from 5 to 30 min before spark ignition was employed to establish whether the mixture is flammable or not. It was shown that duration of the waiting time before spark ignition affected significantly the values of the flammability limits especially at higher temperatures and for rich mixtures. Additionally, the effect of the presence of water vapour in the fuel–air mixture was investigated.