YMgNi(4)H(4) and GdMgNi(4)H(4) hydrides were studied from first principles. Starting from the experimental crystallographic values, the Equation Of State (EOS) of the intermetallics as well as the hydrides have been established. Considering the two possible structures reported for such pseudo Laves phase hydrides (i.e. cubic and orthorhombic), the relative stabilities of the hydrides have been compared. The orthorhombic configuration is confirmed for REmgNi(4)H(4) (RE = Y, Gd) as seen experimentally. Nevertheless, in a larger cell (i.e. REMgNi(4-x)Al(x) solid solution), it has been shown experimentally that the hydride remains cubic, and the present calculations show that the hydrogen atoms have to be considered within the [RENi(3)] tetrahedrons. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.