Soot formation during partial oxidation is a major issue for hydrogen production from liquid hydrocarbon fuels. Measurements were made to investigate the sooting behavior of diesel fuel under variation of the main operating parameters temperature (T = 800 to 1300 degrees C), pressure (p = 1 to 3 bar), equivalence ratio ((P =1 to 3), and steam ratio (H2O/C = 0.2 to 0.6) at constant residence time. The experimental setup was a perfectly stirred/plug flow reactor (PSR/PFR system) providing conditions close to reality. The study proves that Soot growth rate strongly depends on temperature, pressure, and equivalence ratio while adding water has a minor effect on soot growth. Experimental results were compared with a kinetic model developed by the Institut Fran ais du Petrole (UP), predicting soot formation during the partial oxidation of liquid hydrocarbon fuels. The calculated amount of soot shows good agreement with the measured data.