Abstract: The invention is directed to a process of preparing the phosphide of aluminum or magnesium comprising reacting the finely divided metal or an alloy of the two metals with yellow phosphorus at a temperature between 300.degree. and 600.degree. C. in an inert gas atmosphere and in the presence of a catalytically effective amount of the element chlorine, bromine, or iodine or a compound of such element with phosphorus, sulfur, hydrogen, ammonium, zinc, or the metal being reacted, both the yellow phosphorus in liquid form and the finely divided metal being fed into the reaction vessel slowly.