Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H-2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H-2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350 degrees C using the "hercynite cycle" exhibits H-2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350 degrees C and reoxidized at 1000 degrees C.