The limiting conditions for sustained burning of materials are studied experimentally using gas burners. Small pool fire configurations are examined to determine the mass flux for a sustained surface diffusion flame (fire point) and the subsequent extinction limit of that flame. The burner results are compared to material data for sustained ignition, and are found to be lower. Material reported values of a critical mass flux are disparate, and burner data show that the critical mass flux can range from about 1 to 50 g/m2s. Previous studies have indicated the results depend on the convective heat transfer coefficient and the heat of combustion of the gases, but until this work no study has been presented to systematically show these dependencies. Three porous gas burners of diameters 25, 50 and 100 mm were used with fuel gases including methane, propane, isobutene, and ethylene mixed with nitrogen to precisely change the mixture heat of combustion. Diffusion flame theory based on a critical flame temperature at extinction is used to explain and correlate data for both limits. It was found that there is no statistical difference between the sustained ignition and extinction limits. A correlation for the critical mass flux is produced with heat of combustion and fuel diameter as sole dependent variables for all the fuels except methane. The results show that no burning is possible below a heat of combustion of 3–4 kJ/g. This is consistent with the European classification system for non-combustibility where the corresponding limit is set at 2 kJ/g.