In situ high pressure-temperature X-ray observations were made on the system of iron, silicate mineral, and water up to 4.9 GPa and 1350 ¿C using a cubic-anvil high pressure apparatus and synchrotron radiation. Above a pressure of 2.8 GPa at temperatures above 550 ¿C, iron reacts with water to form iron hydride and iron oxide. The chemical composition of the iron hydride formed is estimated to be FeH0.3--FeH0.4. This result implies that if water was contained together with iron and silicates at sufficient depth within the primordial Earth, iron should have transformed into iron hydride. The density of iron hydride thus formed can explain a large portion of the density deficit of the Earth's core. These results show that hydrogen is likely a component of the Earth's core. ¿ American Geophysical Union 1995