Subcritical and supercritical water gasification of petroleum coke and asphaltene was performed at variable temperatures (350-650 ℃),feed concentrations (15-30 wt％) and reaction times (15-60 min).Nickel-impregnated activated carbon (Ni/AC) was synthesized as a catalyst for enhancing syngas yields at optimal gasification conditions (650 ℃,15 wt％ and 60 min).Structural chemistry of precursors and chars developed at different gasification temperatures was studied using physicochemical and synchrotron-based approaches such as carbon-hydrogen-nitrogen-sulfur (CHNS) analysis,thermogravimetric and differential thermogravimetric analysis (TGA/DTA),scanning electron microscopy (SEM),Fourier-Transform Infrared spectroscopy (FTIR),Raman spectroscopy,X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS).Asphaltene testified to be a better precursor for catalytic hydrothermal gasification leading to 11.97 mmol/g of total gas yield compared to petroleum coke (8.04 mmol/g).In particular,supercritical water gasification using 5 wt％ Ni/AC at 650 ℃ with 15 wt％ feed concentration for 60 min resulted in 4.17 and 2.98 mmol/g of H2 from asphaltene and petroleum coke,respectively.Under the same conditions,the respective CH4 yields from catalytic gasification of asphaltene and petroleum coke were 2.54 and 1.07 mmol/g.Nonetheless,asphaltene also seemed to an attractive feedstock for the production of highly aromatic chars through hydrothermal gasification.