Very small amounts of air have been observed to oxidise the surface of the ternary hydride Mg2NiH4. This leads to a profound color change from orange to brown and to a sealing of the surface with respect to desorption, which can be mistaken for a stability increase of the bulk hydride. An unoxidised sample starts to desorb significant amounts of hydrogen when it is heated above 450 K, as is expected from the van't Hoff relation for the hydride. A sample with a slightly oxidised surface may need to be heated above 700 K before hydrogen is released. The Subsequent hydrogen desorption is, on the other hand, very rapid at this elevated temperature. (C) 2004 Elsevier B.V. All rights reserved.