Corrosion behaviour of mild steel in 1M Sulphuric acid with Cloxacillin as corrosion inhibitor has been studied by using weight loss, Potentiodynamic polarization, Electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance spectroscopic studies. All these techniques reveal that inhibition efficiency increases with the increase in the concentration of inhibitor. Scanning electron microscopy (SEM) was carried out to characterize the surface morphology of the metal. Polarization studies indicated that inhibitor behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A chemoffice 3D simulation technique was used to run the quantum mechanical analysis and established correlations between different types of descriptors and measured corrosion inhibition efficiency for inhibitor. The quantum chemical analysis substantiates the inhibition efficiencies of the compound determined by electrochemical methods. © 2013 University of Manchester and the authors.