Many mathematical models have been developed for predicting the temperature rise during the regeneration of diesel particulate filters (DPFs). This requires assuming the magnitude of the temperature gradient normal to the flow in the inlet channels in the soot layer. We report here the first experimental measurement of this value. The experiments revealed that the maximum soot layer temperature occurred at the gas-soot interface. At soot loading of 10 g/L or less the temperature differences across the soot layer are rather small (usually less than 10 degrees C). Higher values may be obtained at higher soot loading. For example, a 40 degrees C difference was observed at a loading of 20 g/L. A surprising finding was that the amplitude of the temperature difference decreased with the increase in feed oxygen concentration. (C) 2011 Elsevier Ltd. All rights reserved.