A model has been derived for the magnesium hydrogenation reaction at conditions close to equilibrium. The reaction mechanism involves an adsorption element, where the model is an extension of the Langmuir adsorption model. The concept of site availability (sigma(s)) is introduced, whereby it has the capability to reduce the reaction rate. To improve representation of sigma(s), an adaptable semi-empirical equation has been developed. Supplement to the surface reaction, a rate equation has been derived considering resistance effects. It was found that close to equilibrium, surface resistance dominated the reaction. Crown Copyright (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.