The rich flammability limits of binary fuel mixtures of hydrogen with methane, ethylene and propane in air were determined experimentally for upward Vertical flame propagation at elevated initial mixture temperatures up to 350 degrees C at atmospheric pressure. A conventional stainless steel flammability test tube apparatus was used. Generally, the flammability limits of hydrogen-methane and hydrogen-propane mixtures obeyed Le Chatelier＇s Rule at elevated temperatures reasonably well (when the corresponding limits of pure fuels were used) if the hydrogen concentration in the mixture was less than 70%. However, the limits of hydrogen-ethylene mixtures deviated very significantly from those calculated using the Rule. It was also found that the rich flammability limit of hydrogen-methane mixtures at initial temperatures higher than 200 degrees C is a function of the residence time. The longer this time and the higher hydrogen concentration in the fuel mixture the smaller was the value of the rich limit. It was suggested that the narrowing of the rich limit is due to surface reactions on the stainless steel wall during the waiting time that tends to change the mixture composition just prior to spark ignition. The flammability limits of these mixtures exposed to longer residence time do not obey Le Chatelier＇s Rule.