The effects of temperature on hydrogen (H-2) production in the alteration of olivine [(Mg,Fe)(2)SiO4] have been investigated under both CO2-rich (0.5 mol/L NaHCO3) and CO2-free conditions at 225-300 degrees C. Under CO2-rich conditions, H2 production rates showed strong temperature dependence, where the rate drastically increased for >275 degrees C, while the rate in CO2-free conditions was less temperature-dependent. At 300 degrees C under CO2-rich conditions, the H-2 generation rate was 3 times faster than that under CO2-free conditions, and a considerable amount of CO2 was simultaneously stored as magnesite [(Mg,Fe)CO3]. Increasing the temperature under CO2-rich conditions promoted olivine consumption, while suppressing both brucite [(Mg,Fe)(OH)(2)] formation and incorporation of Fe(II) into secondary minerals, thus facilitating Fe(II) release and oxidation to produce H-2. It has been revealed for the first time that H-2 production during water-olivine-CO2 reactions is markedly accelerated via high-temperature-facilitated Fe(II) release. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.