Hydrogen-rich syngas production via diesel steam reforming (DSR) is of great interest because of the high H-2/CO ratio and available external heat sources. In this study, catalysts (particle size (d(p)) = 3-5 mm) containing 10 wt % Ni were prepared through wet impregnation, and their performance was evaluated in a semipilot fixed-bed reactor (Phi = 42 mm). The catalyst was doped with La, which resulted in excellent catalytic performance under different operating conditions. The apparent activation energies of the catalysts for the model diesel (n-dodecane) steam reforming increased in the order Ni/Ce0.75La0.25O2-delta-gamma-Al2O3 (17 kJ/mol) < Ni/CeO2-gamma-Al2O3 (18 kJ/mol) < Ni/gamma-Al2O3 (27 kJ/mol). The model DSR over Ni/Ce0.75La0.25O2-delta-gamma-Al2O3 showed product stability and produced a high H-2 content (32 vol %). Fresh and spent catalysts were characterized by X-ray diffraction, N-2 adsorption-desorption isotherms, H 2 -temperature programmed reduction, inductively coupled plasma-mass spectrometry, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and thermogravimetric analysis. The origin of the oxygen vacancies in the La-doped catalyst was elucidated using density functional theory calculations. The improvements in the activity and stability of the Ni/Ce0.75La0.25O2-delta-gamma-Al2O3 catalyst are attributed to (1) the enhancement in the oxygen release/storage capacity, (2) the stabilizing interaction between Ni and the surface La2O3 phase, and (3) the small size (11 nm) and uniform distribution of Ni particles. Out of the tested catalysts, the spent Ni/Ce0.75La0.25O2-delta-gamma-Al2O3 catalyst showed the lowest coke deposition (5.8 mg/g-cat.) and the least severe Ni agglomeration after 10 h of testing because of its superior ionic mobility and oxygen storage capacity.