The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. To clarify the chemical interaction between MgH2 and Nb2 O5 , the mechanochemical reaction products of a composite mixture of MgH2 +0.167 Nb2 O5 was monitored at different time intervals (2, 5, 15, 30, and 45 min, as well as 1, 2, 5, 10, 15, 20, 25, and 30 h). The study confirms the formation of catalytically active Nb-doped MgO nanoparticles (typically Mgx Nby Ox+y , with a crystallite size of 4-8 nm) by transforming reactants through an intermediate phase typified by Mgm-x Nb2n-y O5n-(x+y) . The initially formed Mgx Nby Ox+y product is shown to be Nb rich, with the concentration of Mg increasing upon increasing milling time. The nanoscale end-product Mgx Nby Ox+y closely resembles the crystallographic features of MgO, but with at least a 1-4 % higher unit cell volume. Unlike MgO, which is known to passivate the surfaces in MgH2 system, the Nb-dissolved MgO effectively mediates the Mg-H2 sorption reaction in the system. We believe that this observation will lead to new developments in the area of catalysis for metal-gas interactions.