The solubility of LiOH in Li2O was measured as a function of temperature and partial pressure of H2O. For these measurements, solid Li2O at temperatures of 973 to 1273 K was equilibrated with moisture in a helium carrier gas stream; monitoring the moisture content in the helium carrier gas allowed determination of LiOH solubility. The slope of the curve for the partial pressure of H2O(g) versus LiOH solubility was observed to be 0.6 and is interpreted as an indication of nonideality in the system. For a given partial pressure of H2O, the solubility of LiOH increased with increasing temperature. As a consequence of these measurements, the solubility of tritium (as LiOT) in a breeding blanket of solid Li2O is expected to be low under anticipated fusion reactor environments [e.g., 0.56 wppm tritium at 1000K for 1 Pa (10−5 atm) T2O].