Because of the dissolution of lighter elements such as sulfur and carbon, the Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit, and is therefore an important constituent of the molten iron outer core, the hydrogen concentration in molten iron has been measured using a specially developed method. From these measurements, metal-magma partitioning of hydrogen in the Earth's primordial magma ocean was determined as a function of temperature and pressure where the reaction occurred. It was found. that if the pressure at the bottom of the magma ocean was above 7 to 8 GPa, most of the H<SUB>2</SUB>O absorbed in this ocean should have reacted with the iron to be transported into the core. This seems almost inevitable after the giant impact that formed the moon, although details of the metal-magma reaction process after the impact are not well constrained.