Massive amounts of waste tires are generated globally posing disposal and recycling challenges. Waste tires were gasified in subcritical and supercritical water to study their degradation behavior during syngas production. The process parameters including temperature (325 degrees C to 625 degrees C), reaction time (15 min to 60 min) and feed concentration (5 wt% to 20 wt%) were optimized to maximize gas yields from waste tires. The highest total gas yields (34 mmol/g), hydrogen yields (14.4 mmol/g) and carbon gasification efficiency (42.6%) were achieved at optimal temperature (625 degrees C), reaction time (60 min) and feed concentration (5 wt%). The application of homogeneous and heterogeneous catalysts improved hydrogen yields from waste tires in the order: Ni/SiO2-Al2O3 (19.7 mmol/g) > Ru/Al2O3 (17.9 mmol/g) > Ba(OH)(2) (16.9 mmol/g) > Ca(OH)(2) (16.7 mmol/g) > Mg(OH)(2) (15.4 mmol/g). The results suggest waste tires as a potential resource for recovery of hydrogen-rich fuel gas through hydrothermal gasification. (C) 2019 Elsevier B.V. All rights reserved.