Summary Results are reported on the action of 5% solutions of ammonium salts on a Portland, a Portland-pozzolana, and a high alumina cement concrete. Ammonium chloride and nitrate solutions had a strong leaching action, acting as dilute acids owing to loss of ammonia by reaction with lime. Strengths of concrete cubes with all three cements fell by at least a half in four years without outward signs of deterioration. Analysis of the specimens showed large reductions in the lime content of the cements. In the action of ammonium sulphate on Portland cement, the expansion caused by formation of calcium sulpho-aluminate is dominant. High alumina cement was unaffected by ammonium sulphate.