The velocity of 10 MHz sound waves in solutions of lithium, calcium and barium in ammonia are presented. The sound velocity in the lithium and calcium solutions exhibits a minimum which had not previously been detected. This minimum is a function of the metal concentration and of the temperature. Previous investigations of the sound velocity in potassium and sodium ammonia solutions did not reveal the presence of such a minimum. It is postulated that the minimum is an indication of compound formation in the solutions. Calculations of the sound velocity for the lithium–ammonia solutions are presented using the scheme of Schroeder and Thompson. Data on the saturation curve of the lithium–ammonia solutions is also presented.