Autothermal reforming is a promising technology to produce syngas from diesel fuel. However, the mixing of liquid diesel fuel with high temperature streams of steam and oxygen presents a challenge: how to avoid formation of ethylene, a likely deposit precursor, in the region upstream of the catalyst bed. This work describes a coupled CFD-kinetics study in the mixing region of a diesel autothermal reformer that considers: (1) an atomizer model to explicitly account for fuel evaporation, (2) n-dodecane as a surrogate diesel fuel, (3) oxygen as the oxidant instead of air. The predictions indicate unacceptable levels of ethylene (>0.1 mol%) will be present at the mixer exit if the mixer gas temperature is greater than similar to 350 degrees C. This temperature is likely to be too low for proper catalyst performance. Thus this analysis suggests that either improved mixer designs or a different choice of catalyst might be required to achieve suitable diesel autothermal reforming performance. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.