Here, we report on the hydriding behaviour, of various Mg Ni alloys in the presence of N-2 gaseous impurities using in situ synchrotron XRD. We find that a N-2 content as low as 0.5 mol.% can completely inhibit the hydriding of pure Mg2Ni whereas the Na-modified equivalent remains relatively unaffected even at N-2 concentrations of up to 5 mol.%. The activated hypoeutectic Mg Ni alloy demonstrates excellent hydriding kinetics compared to the as-cast counterpart under 1 mol.% of N-2. Surface analysis shows it is the electropositive nature of a Na-modified Mg Ni surface that facilitates selective adsorption of N-2 while leaving metallic Ni available for H-2 adsorption/dissociation. The results are important in designing Mg Ni alloys with tailored properties that are suitable for use in hydrogen storage/purification/separation applications. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.