The inhibitor effect of vanillin (4-hydroxy-3-methoxy-benzaldehyde) and protocatechualdehyde (3,4-dihydroxy-benzaldehyde) in hydrochloric acid medium on steel with known composition has been investigated. It has been determined using, weight loss, polarization and impedance methods that the corrosion rates decrease and percentage inhibition efficiencies and surface coverage degrees increase with increasing additive concentration. On the other hand inhibition efficiencies are seen to increase with decreasing in temperature. The inhibitors appear to function through the Langmuir adsorption isotherm. Polarisation studies indicate the compounds studied to be mixed inhibitors. All the methods employed are in reasonable agreement. (C) 2003 Elsevier B.V. All rights reserved.