Developing a better ceramic breeder (Li-containing oxide) is a key challenge for realizing fuel-self-sufficient fusion reactors. Ceramic breeder pebbles of hyper-stoichiometric lithium metatitanate, Li2+x TiO3+y (Li/Ti > 2), have been developed for a demonstration fusion reactor as high Li density enhances fuel tritium production. Previous studies have reported that Li loss by vaporization at high temperatures was largely enhanced by the increase in Li/Ti ratio and environmental moisture concentration. Minimizing the Li loss is a key issue for sufficient tritium breeding and reduced corrosion of structural steel. However, a suitable environmental parameter for hyper-stoichiometric Li2+x TiO3+y pebbles has not yet been determined because of the unavailability of thermodynamic data. Herein, a solid/gas equilibria for Li2.12(2)TiO3+y was investigated by measuring vapor pressures using atmosphere controllable Knudsen cell high temperature mass spectrometry. The enhanced thermodynamic activities of Li and Li2O in Li2.12(2)TiO3+y in comparison with those in stoichiometric Li2TiO3 were obtained as functions of temperature and oxygen concentration. The equilibrium constants were used to achieve the optimum moisture concentration that can suppress the vapor reactions in a cylindrical breeding zone with inhomogeneous temperature distribution.