Abstract It has been known that water vapor is released from ceramic breeder materials into the purge gas due to desorption of adsorbed water under dry atmosphere and due to the water formation reaction under hydrogen atmosphere. However, an effect of water vapor in the purge gas to Li mass loss has not been understood. In this study, mass loss of Li2TiO3 (NFI) and Li4SiO4 (FzK) under hydrogen atmosphere (1000Pa H2/Ar), and mass loss of Li2TiO3 (NFI) and Li2TiO3 with additional Li which is in a developmental stage (JAEA) under water vapor atmosphere (50Pa H2O/Ar) were compared, respectively. It was found that under hydrogen atmosphere Li mass loss of Li4SiO4 and Li2TiO3 is same degree although the amount of water vapor released from Li4SiO4 is larger than that from Li2TiO3. It was clarified with regard to Li2TiO3 that Li mass loss in water vapor atmosphere is larger than that in hydrogen atmosphere. Mass loss of Li2TiO3 with additional Li (JAEA) was larger than that of Li2TiO3 (NFI). It was observed by X-ray analysis that Li deposits formed on the inner wall of the quartz tube contain Li2SiO3.