A Procedure is described for calculating the lean flammability limits of fuel-diluent mixtures in air over a wide range of fuel-diluent combinations and for different initial mixture temperatures. Good agreement is shown to exist between the predicted values of the limits with the corresponding experimental values for some common gaseous fuels that include CH4, C2H6, C2H4, C3H8, C4H10, H-2, and CO and the diluents CO2, N2, He, and Ar over the temperature range of - 60-degrees-C up to 400-degrees-C.