The corrosion behaviour of 304 stainless steel (SS) in 0.5 M H2SO4 solution was studied using potentiodynamic and galvanostatic polarization techniques. Three anodic peaks were observed in the potentiodynamic anodic polarization curves before oxygen evolution. The effects of acid concentrations and voltage scan rate were studied. The inhibitive effect of 4-substituted pyrazole-5-ones toward the corrosion of 304 SS in 0.5 M H2SO4 were studied. The inhibition action of these compounds was assumed to occur via adsorption on the steel surface through the active centres contained in their structure. The mechanism of inhibition was interpreted on the basis of the inductive and mesomeric effects of the substituents. There is a good agreement between the percentage inhibition efficiencies calculated from both techniques. The inhibition efficiencies increase with increasing the inhibitor concentrations. (C) 2003 Elsevier B.V. All rights reserved.