The present study reports a comprehensive analysis regarding the formation of Mg-Nb combined active rock salt nanoparticles (MgxNbyOx+y) in MgH2/Nb2O5 composite hydrogen storage system. This phase exhibit close crystallographic features to MgO and, hence was overlooked for years until recently. In order to explore the evolution of this phase in-situ in a MgH2-Nb2O5 hydrogen storage system, Nb2O5 was mixed with MgH2 in various concentrations and the mechanochemical reaction products were monitored through 8 different compositions in three sets of reaction times, 1 h, 10 h and 30 h (totally 24 samples). Our study confirms the formation of rock salt type MgxNbyOx+y nanostructure over a wide compositional range in the MgH2 + nNb(2)O(5) (n = 0.083, 0.10, 0.123, 0.167, 0.25, 0.50, 1.0 and 1.5) system. In contrast to MgO, which is a well-known barrier layer in MgH2 system, the Nb dissolved MgO phases are shown to promote the hydrogen desorption performance of MgH2. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.