We studied the production of hydrogen by the catalytic steam reforming of sunflower oil, a renewable resource that is already used as feedstock for producing biodiesel. The study was performed in a fixed-bed reactor with a commercial nickel-based catalyst for steam-reforming naphtha. Steam-to-carbon (S/C) ratios of 3, 6, and 9 and catalyst temperatures from 550 to 880 degrees C were tested at a constant space velocity (MC1HSV) of 410 mol(carbon) h(-1) L-catalyst(-1), equivalent to a gas hourly space velocity (G(C1)HSV) of 9150 h(-1). Sunflower oil was completely converted to hydrogen, methane, and carbon oxides, except for the runs performed at the lowest temperatures and an S/C ratio of 3. The hydrogen yield ranged from 72% to 87% of the stoichiometric potential, depending on the steam-to-carbon ratio and the catalyst temperature, which governed the equilibrium among gas species. Thermal cracking of the oil in the gas-phase was a competing process during steam reforming, especially above 650 degrees C where it converted fatty acids to a wide array of products, including ethylene and aromatics. Cracking products were subsequently converted to hydrogen and carbon oxides by the catalytic steam-reforming reaction. A gradual deactivation of the catalyst by carbon deposition was observed in long-duration reforming experiments, although catalyst activity was restored after steaming at 850 degrees C for 3 h.