The solubility of hydrogen in liquid alkali metals useful as nuclear reactor coolants varies significantly with pressure and temperature. The solubility of hydrogen in eutectic sodium-potassium mixture (NaK-78) was determined at six temperatures from 300 to 704°C and at pressures below one atmosphere. For unsaturated solutions, the solubility depended on the square root of the hydrogen pressure and varied only slightly with temperature. At temperatures of 300 to 400°C, enough hydrogen could be dissolved at pressures below one atmosphere to result in precipitation of a metal hydride. Precipitation-decomposition pressures were consistent with the literature. The addition to NaK-78 of 1 to 4 at.% lithium considerably diminished the hydrogen activity and resulted in precipitation at lower hydrogen pressure. A mass action model is postulated to explain the phenomena. It is suggested that in liquid-alkali-metal mixtures, dissolved hydrogen exists largely in the form of undissociated metal-hydride molecules or ion pairs, with the different metals combining with the hydrogen in proportion to their concentration and their affinity for hydrogen.