Hydrogen production via steam and autothermal reforming of soybean waste frying oils (WFOs) is thermodynamically investigated via the Gibbs free energy minimization method. The thermodynamic optimum conditions are determined to maximize hydrogen production while minimizing the methane and carbon monoxide contents and coke formation. Equilibrium calculations are performed at atmospheric pressure over a wide range of temperatures (400-1200 degrees C), steam-to-WFO ratios (S/C: 1-15) and oxygen-to-WFO ratios (O/C: 0.0-2.0). The baseline case used for the study considers soybean WFO after 8 h of use (WFO8). The influence of frying time on the performance of reforming reactors is also discussed. The results show that the optimum conditions for steam reforming can be achieved at reforming temperatures between 650 degrees C and 850 degrees C and at a steam to carbon molar (S/C) ratio of approximately 5. The recommended operation conditions for the SR of WFO8 are proposed to be T = 650 degrees C and S/C ratio = 5. Under these conditions, a hydrogen yield of 169.83 mol/kg WFO8 can be obtained with a CO concentration in the SG of 3.91% and trace CH4 (0.03%), without the risk of coke formation. Hydrogen production from autothermal systems can be optimized at temperatures of 600-800 degrees C, S/C ratios of 3-5, and O/C ratios of 0.0-0.5. Under these conditions, thermoneutrality is obtained with O/C ratios of 0391-0.455. The recommended thermoneutral conditions are S/C = 5, T = 600 degrees C and O/C = 0.453. Under these conditions, 146.45 mol H-2/kg WFO8 can be produced with only 2.89% CO and 0.06% CH4 in the synthesis gas. The effect of frying time of soybean WFO on hydrogen productivity is shown to be negligible. (C) 2013 Elsevier Ltd. All rights reserved.