A mass-transfer study was carried out in water and water/oil flow in a large diameter flow loop by a limiting current density technique using oxidation of Fe(CN)(6)(4-) in 1 hi NaOH. High-frequency and high-magnitude mass-transfer coefficients were found by potentiostatic measurements and may account for the corrosion rate enhancement in water/oil flow. The effect of oil droplets on the mass-transfer enhancement is highlighted and a mechanism of mass-transfer coefficient oscillation is proposed.