The hydration phase diagram for sodium cobalt oxhydrate, Na(0.3)CoO(2)(.)yH(2)O (y = 0, 0.6 and 1.3), was determined as a function of relative humidity at 298 K It is found that greater than 75% relative humidity is needed for complete hydration of anhydrous Na0.3CoO2 to the superconducting phase Na(0.3)CoO(2)(.)1.3H(2)O. Dehydration studies show that a minimum of 43% relative humidity is needed to maintain the stability of the fully hydrated superconducting phase. The intermediate hydrate, Na(0.3)CoO(2)(.)0.6H(2)O, is stable between 10 and 50% relative humidity on hydration, and 35 and 0% relative humidity on dehydration. (c) 2005 Elsevier Ltd. All rights reserved.