As strong reducing agents, alkali metals including Li, Na and K enable the electroless synthesis of magnesium nanoparticles. In this investigation, we determined the effects of such alkali metals on the properties of magnesium at the nanoscale. The magnesium nanoparticles were successfully synthesised by directly reducing di-n-butylmagnesium with an excess amount of alkali-metal naphthalenide in order to coat individual magnesium nanoparticles within an alkali matrix. Upon heat treatment under hydrogen pressure a perovskite structure MMgH3 was formed on Na- and K-coated magnesium nanoparticles under mild conditions but was absent on the Li-coated materials leading to LiH only. These perovskite structures were found to have no significant influence on the thermodynamics of magnesium. Compared to ball-milled MgH2, these materials showed lower hydrogen absorption plateau pressure (similar to 0.05 MPa). However, the magnesium particles generated with potassium had fast hydrogen kinetics and low reaction enthalpy due to their small size. More remarkably these coated magnesium nanoparticles remained stable upon cycling which provides a new route to stabilise magnesium nanoparticles. Crown Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.