The crystal structures and hydrogenation behavior of (La 1-x Mg x ) 3 Al (x = 0.1, 0.2, 0.3, and 0.4) alloys were investigated. It was found that the alloys with x = 0.1 and 0.2 consist of La(Mg, Al), La, and a novel phase. The novel phase was determined as La 2 Al. It is shown that the amount of La 2 Al decreases as the Mg content increases. When x increases to 0.3, only La(Mg,Al) and a small amount of La 2 Al exist. When x is 0.4, La 2 Al phase disappears and the alloy contains both La(Mg,Al) and La(Al,Mg) 2 Laves phase. The (La 0.9 Mg 0.1 ) 3 Al and (La 0.7 Mg 0.3 ) 3 Al alloys can be decomposed into LaH 3 , MgH 2 , and La 2 Al 5 by the absorption of hydrogen at 473 K.