The hydrogen storage characteristics of thin film MgX (X=Sc, Ti, V, Cr) compounds were investigated electrochemically. The successful preparation of these metastable, crystalline single-phase, MgX compounds was achieved by means of electron-beam deposition at room temperature. The reversible hydrogen storage capacity of these compounds is excellent and up to six times higher than commercial AB5-type materials. The gravimetrical storage capacities of these new materials were determined to be 1790 mAh/g for Mg80Sc20, 1750 mAh/g for Mg80Ti20, 1700 mAh/g for Mg80V20, and 1325 mAh/g for Mg80Cr20, corresponding to 6.7, 6.5, 6.4, and 4.9 wt % H, respectively. The hydrogen absorption and desorption kinetics are profoundly influenced by the element X incorporated in the MgX compound. Galvanostatic measurements show that the rate capability of the Sc- and Ti-containing compounds is significantly better than that of the V- and Cr-containing compounds. Isotherms of these systems are obtained using galvanostatic intermittent titration technique, revealing that the equilibrium potential of the main charge/discharge plateau, apart from the Mg80Sc20 compound, only slightly depends on X in MgX. The electrochemical measurements show that low-cost Ti is an excellent substitute for the expensive Sc in MgSc, without introducing detrimental effects.