A method of preparing an ammonia borane compound selected from the group consisting of: ammonia borane, ammonia alkyl borane, ammonia aryl borane and mixtures thereof, the method including the steps of: a) incorporating a reaction mixture into a pressure vessel, the reaction mixture including anhydrous liquid ammonia and a boron containing compound that can react under pressure with the liquid ammonia to form the ammonia borane compound; and, b) causing the reaction mixture to warm from a first temperature greater than or equal to −33° C. to a second temperature under pressure to form the ammonia borane compound.