Metallographic information has been used to support the kinetic analysis and the relative experimental findings in the phase transformation from Mg to MgH2, by absorption of hydrogen gas. In particular the method provides to be able to provide detailed information on the role of localized features like catalyst particles and defects present in the sample, which is obtained from ball milled MgH2 enriched with 5wt% Fe. In this work we have compared the kinetics of the hydrogenation reaction carried out at two different temperatures while keeping constant the thermodynamic force driving the reaction. This last was achieved by carefully controlling the hydrogen gas pressure. Despite this fact, the sample microstructure shows a marked effect of the temperature on the nucleation mechanism. In particular we have noticed that the density of sites active for nucleation is higher at lower temperature. Instant nucleation deduced by the kinetic analysis was confirmed by comparing the microstructure of samples at different reaction stages. © 2010 Materials Research Society.