N, N′-Diphenylthiourea (DPTU) and N-naphthyl- N′-phenylthiourea (NPTU) synthesized in our laboratory, were tested as inhibitors for the corrosion of cold-rolled steel in 0.5 M H2SO4 by weight loss and electrochemical measurements. The studies clearly reveal that when we substitutes a phenyl group in N, N′-diphenylthiourea (DPTU) by naphthyl group to obtain N-naphthyl- N′-phenylthiourea (NPTU), the inhibition efficiency increases from 80 to 96% at 2 × 10−4 M. Polarization curves show that NPTU acts as mixed type inhibitor whereas DPTU predominates as cathodic inhibitor. Changes in impedance parameters (charge transfer resistance, R t, and double layer capacitance, C dl) were indicative of adsorption of DPTU and NPTU on the metal surface, leading to the formation of protective films. The degree of the surface coverage of the adsorbed inhibitors is determined by ac impedance technique, and it was found that the adsorption of these inhibitors on the cold-rolled steel surface obeys the Langmuir adsorption isotherm. The effect of the temperature on the corrosion behavior with addition of 10−4 M of DPTU and NPTU was studied in the temperature range 20–50 °C. Results show that the rate of corrosion of mild steel increased with increasing temperature both in the presence of inhibitors and in their absence. Activation energies in the presence and absence of DPTU and NPTU were obtained by measuring the temperature dependence of the corrosion current. The reactivity of these compounds was analyzed through theoretical calculations based on density functional theory to explain the different efficiency of these compounds as corrosion inhibitors. [ABSTRACT FROM AUTHOR]