The reaction of the gallium analog of β″-A123 with moisture was investigated. This material in finely divided form reacts rapidly with moisture from humid atmospheres to pick up approximately 1 mole of water for each gram-atom of sodium. The water uptake is accompanied by an expansion in the c axis of 11-12%. Monocrystals of the β″-gallate do not show measurable water pickup, and particles of intermediate sizes show intermediate degrees of pickup. The size effect is explained as peripheral attack of water on the crystal, which ceases after a fixed penetration depth. An association of water molecules with the sodium ions to form a complex such as (NaH2O)+, which increases the coordination of the site, is suggested as the reaction mechanism. © 1977, The Electrochemical Society, Inc. All rights reserved.