Erosion-corrosion of 12 alloys in phosphoric acid waste water was studied by the Bureau of Mines. Tests were conducted on oblate spheroids or cylindrical specimens in 12- and 60-cm-diam ball mills by grinding phosphate rock in gypsum pond water with a starting ph of 1.6. For comparison, tests were conducted while grinding phosphate rock in plain water. Static corrosion tests in gypsum pond water also were conducted. For all alloys, the wear by erosion- corrosion (milling with gypsum pond watrer) was greater than the sum of erosion (milling with water) plus static corrosion. Erosion- corrosion wear was about 1,500 to 3,000 mils per year (mpy) for the alloyed white cast irons, 1,000 to 1,400 mpy for commercial steel ball material and type 316 stainless steel, and 600 to 900 mpy for nickel-base alloys and a high-manganese, nitrided stainless steel. Alloy costs per unit of phosphate rock processed favor a low-alloy, high-carbon steel or a high-manganese, nitrided stainless steel.