Macrocyclic compounds constitute a potential class of inhibitors. In an attempt to develop effective corrosion inhibitor we have synthesized a macrocyclic compound namely 2,3,9,10-tetramethyl-6,13-dithia-1,4,5,7,8,11,12,14-octaaza-cyclotetrade ca-1,3,6,8,10,13-hexaene (MTAH) was synthesized to study the corrosion inhibitive effect on pickling of mild steel (MS) in 20% H2SO4 at 95 degreesC. The synergistic effect of this compound was studied by weight loss, potentiodynamic polarization, electrochemical impedance and hydrogen permeation studies. The results of investigations in the presence of potassium iodide (KI) as synergent have shown enhancement in inhibition efficiency (IE). Potentiodynamic polarization studies showed that MTAH is mixed in nature. (C) 2002 Elsevier Science B.V. All rights reserved.