The present work studies the flammability limits of combustible–diluent–air mixtures at 25 °C and 1 atm pressure. The diluent species, considered in this study, were nitrogen and carbon dioxide. A factor called kFL was obtained from mass and energy conservation, this factor relates the flammability limit of a combustible–air mixture to that of a combustible–diluent–air mixture. The factor kFL varies with diluent concentration. Nevertheless, it was shown that an average value of this factor (kFLav) can represent, with good accuracy, the variation of the flammability limits with diluent concentration. An empirical method to determine the flammability limits of combustible–diluent–air mixtures was developed. The method approximates kFLav by means of correlations. For the total set of experimental data, it was found that, for dilution with N2 and CO2 respectively: (a) at the LFL the Average of the relative errors (AARE) were 4.02% and 7.01%, the squared correlation coefficients (R2) were 0.9833 and 0.9660; and (b) at the UFL the AAREs were 3.91% and 5.57%, the R2 were 0.9920 and 0.9792. At the Fuel Inertization Point (FIP) there is an inert concentration above which the mixture is non-flammable. A method was developed to determine the diluent mole fraction in the combustible–inert mixture at the FIP. The AAREs of 0.82% and 1.95% were obtained for dilution with N2 and CO2, respectively.