The influence of (6R,7R)-7-[[(2R)-2-amino-2-phenylacetyl]amino]-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (APMC) on corrosion and hydrogen permeation through stainless steel 304 in 3.5%NaCl has been studied using weight loss measurements and various electrochemical techniques. The compound is found to be more inhibitive in sea water medium. Potentiodynamic polarization studies clearly indicate that APMC behaves as a cathodic inhibitor. Hydrogen permeation studies and AC impedance measurements also prove an improved performance of the compound in 3.5% NaCl. The adsorption of this compound on the stainless steel surface obeys Temkin's adsorption isotherm. © 2014 University of Manchester and the authors.