The flammability limits of hydrogen-diluent fuel mixtures in air for upward and downward flame propagation at ambient pressure were determined experimentally. A wide range of diluent concentrations were examined, up to 95% by volume in the fuel mixture. The diluents employed were helium, argon, carbon dioxide and nitrogen. The effects of changes of the initial mixture temperature on the flammability limits were determined for fuel mixtures containing 50% diluent by volume. It was observed that helium was more effective than argon at narrowing the flammable range of hydrogen mixtures. The value of the lean flammability limit was increased to a greater extent by the addition of helium than carbon dioxide despite the low heat capacity of helium. Values of the flammability limits predicted using the constant adiabatic flame temperature concept are in generally good agreement with the experimentally obtained values. The agreement between the two sets of values was better for the rich flammability limits than the corresponding lean flammability limits and was particularly poor for lean mixtures involving high concentrations of helium.