When MgH2 is processed under relatively mild milling conditions it exhibits a two-step thermal decomposition process where loss of hydrogen is associated with each endothermic event. To date, no causal link between the two thermal events and the material has been established and no absolute description of the mechanism involved in the two hydrogen desorption steps has been determined. To unravel the decomposition mechanism of the milled hydride, we propose RT and in-situ small angle scattering measurements to decouple the effects of microstructure and phase behaviour in the dehydrogenation of MgH2; a complementary proposal covering neutron diffraction measurements has been submitted. Here we will determine the particle surface scattering properties of intermediate partially hydrogenated (MgH2-x) phases that evolve during the ¿low temperature¿ (LT) and ¿high temperature¿ (HT) endotherms.