The effect of Nb 2O 5 concentration on the kinetics of the magnesium hydrogen sorption reaction at 300 °C and 250 °C is investigated. Absorption and desorption kinetics of nanocrystalline magnesium catalyzed with 0.05, 0.1, 0.2, 0.5, and 1 mole% Nb 2O 5 are determined at 300 and 250 °C. Fastest kinetics are obtained using 0.5 mole% Nb 2O 5. At 300 °C, absorption and desorption of 7 wt.% of hydrogen are facilitated in 60 and 90 s, respectively. At 250 °C, more than 6 wt.% are absorbed in 60 s and desorbed again in 500 s. Respective reaction rates and activation enthalpies are calculated. The activation energy for desorption varies exponentially with Nb 2O 5 concentration and reaches a limit of 61 kJ/mole at 1 mole%. The rate-limiting step for a particular parameter set is deduced by fitting the experimentally obtained transformed phase fraction and time data with analytical kinetics rate expressions. The results show that there is a change in the rate-limiting step with catalyst content.