The autothermal reforming (ATR) performance of diesel, biodiesel, diesel blended with 25% biodiesel, and diesel blended with both 25% biodiesel and 10% ethanol was investigated under different operating environments in a single-tube reformer with rhodium/zirconia wash-coated ceramic monoliths. A customized nozzle integrated with a microsize porous device was designed to finely atomize the input heavy hydrocarbon fuels and enhance the homogeneous mixtures of hydrocarbon fuel with steam and air. The optimum operating conditions with high syngas yields and carbon-free content for ATR of each studied fuel were identified, from both thermodynamic analysis and experimental measurements. After establishing the test points for the reformation of the studied fuels under the same initial operating environment [total O/C = 1.47, S/C = 0.6, and gas hourly space velocity at standard temperature and pressure (GHSV) = 34 120 h(-1), at 950 degrees C reformer temperature] with the goal of achieving the same rate of syngas production, three series of experimental tests were conducted to analyze the impacts of input air reduction, reformer temperature reduction, and GHSV changes on the effluent species from the reactor. A direct photoacoustic microsoot meter was applied to quantify the dynamic evolution of carbon formation under these changing conditions to determine the carbon boundaries for the ATR of each fuel. The correlations between ethylene and carbon formation were observed, and the fundamental understanding of their relationship was further investigated.