The H-sublattice ordering and the effect of local atomic relaxations due to H interstitial atoms on the structural stability of hexagonal hydrogen-M solid solutions (M = Sc, Ti, Y, Zr) is studied by means of the density functional theory. A first-principle based model is presented which, by considering the relaxation of the host metal cell, yields for the first time a suitable explanation for the observed chain-like short range ordering through a coherent stress field along the chain as well as its limitation to only six hcp metals, namely Sc, Y, Ho, Er, Tm, and Lu, and the absence of a mono-hydride phase in the same elements. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.