Sliding wear tests were conducted to study the consequences of extremely severe wear of stainless steel component in high-temperature, high-purity water. The tests were performed in a normal load range of 1.88 × 10 3 − 1.18 × 10 4 N at a sliding speed of 50 m s −1 , using stationary and rotating disc specimens on an industrial scale. Investigations were focused on the effects of load and water temperature on the wear rate, the mechanical behavior of the stainless steel surface and the water chemistry changes caused by continuous severe wear at elevated temperature. The wear rate increased exponentially with increasing load, and was two times greater at 260°C than at room temperature. The ratio of wear rate of the rotating specimen to the stationary specimen decreased exponentially with increasing load. The wear particle sizes were distributed widely up to 4 mm, and most wear debris consisted of thin flakes with scratch marks on the surface. A transformation layer 50 μm deep was observed in cross-sections of the debris, within which oxide incorporation and selective dissolution of metal components took place in high- temperature water. The bare surface of the stainless steel, created continuously by severe wear, was oxidized in high-temperature water and caused the water chemistry change such as the consumption of dissolved oxygen and the evolution of dissolved hydrogen.