Hydrides of intermetallic compounds are used as hydrogen accumulating systems. They also exhibit catalytic activities in reactions involving hydrogen, as has been reported in numerous publications [1][2][3][4][5][6]. Until the mid-1980s, two methods for the preparation of intermetallic hydrides were known, namely, by hydrogenation of alloys obtained by the pyrometallurgical process [1-3] and via organomagnesium compounds [4]. The drawbacks of the former method are the high energy consumption, the necessity of lengthy treatment of systems by hydrogenation-dehydrogenation cycles to ensure complete interaction of the metal with hydrogen, and the low specific surface area. The drawbacks of the latter method are related to the poor practicability of processes involving organometallic compounds in industry. The hydride preparation method using so-called mechanical alloys obtained by mechanochemical activation of a mixture of two metals is free from these drawbacks [5]. These systems are readily hydrogenated without additional treatment and have a rather extensive specific surface area, and the energy expenditure for mechanical activation is much lower than in the pyrometallurgical method. Apart from known hydrides, unknown compounds have been prepared by this method [6,7].We together with researchers from the Institute of Solid-State Chemistry and Mineral Processing, Siberian Division of the RAS, employed this method to prepare catalytic systems based on intermetallic hydrides of magnesium and iron group metals [8][9][10][11][12], some of which are highly selective in the hydrogenation of acetylenic and diene hydrocarbons to olefins. The major drawback of this method is the necessity of hydrogenation of the mechanical alloy under rigorous conditions. When the optimal temperature-pressure ratio is not maintained, hydrogenation takes a long time. To eliminate these drawbacks, we developed a method for the synthesis of intermetallic hydrides involving the mechanochemical treatment of alloys or metal mixtures at high hydrogen pressures [13]. As a further development of this method, we studied the effect of ammonia on the mechanochemical synthesis of intermetallic hydrides at high gas pressure.The mechanical activation was carried out in an AGO-2 planetary mill (developed at the Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division of the RAS) with steel balls 5-10 mm in diameter at a drum rotation frequency of 10-17 rps and a drum volume of 150 cm 3 . The atmosphere in the drums was created in a setup that provided a gas pressure of up to 10 MPa. Ammonia was pumped into a drum (prewashed several times with hydrogen) in an amount of 5 vol % relative to hydrogen; subsequently, hydrogen was pumped in to reach the specified pressure.The hydrogen content of the samples was determined using two volumetric methods, namely, by measuring the volume of hydrogen evolved on heating and on reaction with an acid. Heating of the magnesium/nickel intermetallic hydride to 673 K was carried out in a reactor with draw-of...