This communication deals with the ex-situ synthesis of rock salt type MgxNb1-xO whose structural characteristics are closely related with MgO. XRD examination of 30 h ball milled MgH2 + Nb2O5 confirms the formation of a rock salt product MgxNb1-xO, which is comparable to the recently reported active catalyst MgxNb1-xO formed in-situ in MgH2 milled with 8 mol.% Nb2O5. It is shown that MgH2 catalyzed with the pre-made 2 wt.% MgxNb1-xO desorbs hydrogen at least 50 degrees C lower than the in-situ 2 wt.% Nb2O5 catalyzed MgH2 with improved reversible absorption. This result highlights that the proposed pathway mechanism on the basis of Nb2O5 catalyst may need further verification and that the addition of the MgxNb1-xO catalyst in a pre-reduced state can offer distinct performance advantages over its in-situ preparation. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.