In this work, Mg50Ni50 hydrogen storage alloy was synthesized by mechanical alloying technique, by using a planetary high energy ball mill (Retsch PM 200). The morphological and microstructural characterization of the powders was performed via scanning electron microscopy and X-ray diffraction. The dehydriding characterization of the composite was performed via a solid gas reaction method at different temperature 313 K, 333 K and 353 K. A new model has been developed, describing pressure-composition isotherms basing on statistical physics treatment. The monolayer model with two types of sites is found to fit very well with experimental data obtained on Mg50Ni50. The parameters involved in the model were determined directly from the experimental data by numerical simulation. The behaviors of these parameters are discussed in relationship with absorption and desorption process. Finally, the different thermodynamical potential functions are derived by statistical physics calculations from the adopted model. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.