Phosphor-bronze pump impellers handling hot feed-water and condensate on a power station have been found to be suffering from a form of attack analogous to dezincification. Micrographic, analytical, and X-ray evidence of the nature and products of the attack is presented. The castings were highly porous, and water penetrating down the pores has led to the formation of particles of copper and stannic oxide deep in the metal. This serious attack has taken place from a machined surface, where the pores were exposed. On surface not machined, attack is not very important, but has produced a number of structure which are not understood.