The inhibitive action of new 4-arylhydrazono-2-pyrazolin-5-one compounds on the dissolution of aluminium in 2M HCl has been investigated by weight loss, thermometric, hydrogen evolution and galvanostatic polarisation techniques. All the methods gave similar results. An increase in the concentration of compounds increases the inhibition efficiency leading to a decrease in the corrosion rate. The activation energy of the dissolution reaction increases as the inhibitor efficiency is increased. The values of free energy of adsorption Delta G(ads)(o) have been calculated and discussed. These compounds act as general type inhibitor and adsorb on both anodic and cathodic sites. Electrocapillary curves support the tendency of inhibitors to adsorb on metallic surfaces.