The spontaneous ignition temperature as defined using the American Society for Testing and Materials (ASTM) methods may not accurately determine the temperature at which carbon will combust when a step change in bed temperature occurs; therefore, an instantaneous ignition temperature is defined. Seven different activated carbons have been heated at various heating rates in three different bed configurations, and the data show that the instantaneous ignition temperature can be significantly lower than the spontaneous ignition temperature. In some cases, the carbon ignited at temperatures 100 degrees C lower than the spontaneous ignition temperature. The results show that ignition temperatures are dependent not only on the carbon source and bed dimensions but also on the rate of heating used in the experiment. The results emphasize the importance of evaluating carbon combustion with experiments that closely resemble the operational conditions of the fixed-bed