The corrosion rate of copper, copper alloys and carbon steel in flowing 3 per cent NaCl solution is found to be mainly determined by mass-transfer phenomena. In the case of copper and its alloys, the corrosion rate at low flow velocities is limited by the removal of corrosion products from the metal surface, providing the supply of oxygen is large and thus not rate-determining. In the case of carbon steel, on the other hand, the supply of oxygen to the metal surface determines the corrosion rate. This is proved by the agreement between experimental corrosion rates and values calculated from cathodic limiting current densities for oxygen reduction. For all metals investigated, a marked increase in the corrosion rate at the transition from laminar to turbulent flow was observed. A recalculation of data in the literature from linear flow velocities to Reynolds' numbers also showed a pronounced coincidence between the Re crit and the appearance of erosion-corrosion. At very high Reynolds' numbers, the corrosion rate of copper and its alloys was found to be virtually independent of the flow velocity. Here, the mass-transfer processes are apparently so rapid, that another reaction step, viz. the ionization of the metal, is rate-determining.