Novel corrosion inhibitors, namely 1-12-[(2-hydroxyethyl)thiolethyl] pyrrolidin-2-one (P5) and {[2-(2-oxopyrrolidin-1-yl)ethyl]thio}acetic acid (P4), were synthesised and tested as corrosion inhibitors for steel in 0.5 M H2SO4. The effects of P4 and P5 are also compared to their initial reactants 1-vinylpyrrolidin-2-one (P1), 2-mercaptoethanol (P2) and mercaptoacetic acid (P3). The study was carried out by weight loss measurements, potentiodynamic polarisation, linear polarisation resistance (R-p) and electrochemical impedance spectroscopy (EIS) methods. The inhibition efficiency increases with the concentration of P5 to attain 89% at 5 x 10(-3) M. We note good agreement between the various methods explored. Polarisation measurements show also that the pyrrolidones act essentially as cathodic inhibitors. The cathodic curves indicate that the reduction of proton at the steel surface is an activating mechanism. P4 and P5 adsorb on the steel surface according to Langmuir adsorption model. Effect of temperature is also studied in the 298-353 K range. Efficiency is explained by the theoretical studies. (c) 2005 Elsevier B.V. All rights reserved.