Equilibrium hydrogen pressures (PH2) were measured for the two-phase system, Li-LiH, for the liquid lithium of dilute solutions of 2.0 × 10−3 to 1.3 × 10−2 atomic ratio (H/Li), and for the liquid lithium controlled by a cold trap at temperatures between 200 and 450°C. From the variation of PH2 with temperature, the apparent heat of solution of hydrogen in liquid lithium was obtained as 34 and 19 kcal/mol for the Li-LiH and for the dilute solutions, respectively. For the liquid lithium controlled by the cold trap at 203°C, the heat of solution was 19 kcal/mol. Using the Sieverts constant, which was obtained experimentally in this work, it was possible to determine the hydrogen concentration in liquid lithium from the PH2. The hydrogen solubility in liquid lithium in the concentration <2 × 10−2 H/Li (corresponding to the temperature lower than 350°C) was estimated from the hydrogen concentration data obtained through the PH2 of the Li-LiH system. Although the cold trap could effectively control the hydrogen concentration in liquid lithium, this solubility was apparently lower than that obtained from the Li-LiH by factors of 2 to 3 at the same temperature. The difference can be attributed to the interactions among hydrogen and other impurities and to compound formations such as Li2H.