The work of Guvendiren et al. on the effects of additives on mechanical milling and hydrogenation of Magnesium Powder which is published in this journal [Guvendiren M, Bayboru E, Ozturk T. International journal of Hydrogen Energy 2004; 29: 491-496] shows excellent experimental work which agrees with previous published work. However they did not explain the right phenomenon which is undergoing during hydrogen absorption on magnesium hydride's system. in this communication it is the objective to distinguish between thermodynamic and kinetics factor effecting hydrogen absorption. It will be shown that the phase rule of thermodynamics will determine the variation of Pressure-composition isotherms at constant temperature during hydrogen absorption in Magnesium or Titanium. This is because the Pressure-composition isotherms at constant temperature is different for a single-phase (e.g. beta-Titanium) than two phases (e.g. delta and epsilon phases). Thus the data of Guvendiren et al. published in this journal (Fig. 5) can be explained by this effect. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.