EIS studies of an imidazoline-based inhibitor under multiphase flow conditions were conducted in a large diameter flow loop system. The Warburg diffusion parameter, b(f) = σω(- 1/2) was used to qualitatively analyze inhibitor films formed on the metal surface. Experimental results show that the inhibitor films become less porous with increase in exposure time and inhibitor concentration. Inhibition performance of the corrosion inhibitor is reduced in slug flow at high rate of turbulence and bubble impact. This study demonstrates that EIS is an effective technique to investigate the inhibition performance of corrosion inhibitors in turbulent flow conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.