4 -phenylazo-3-methyl-2-pyrazolon-5-one and three of its derivatives have been investigated as corrosion inhibitors for C-steel in 2 M hydrochloric acid solution using weight-loss and galvanostatic polarization techniques. The efficiency of the inhibitors increases with the increase in the inhibitor concentration but decreases with a rise in temperature. The conjoint effect of the pyrozolone derivatives and KBr, KSCN and KI has also been studied. The apparent activation energy (E-a*) and other thermodynamic parameters for the corrosion process have also been calculated. The galvanostatic polarization data indicated that the inhibitors were of mixed-type, but the cathode is more polarized than the anode. The slopes of the cathodic and anodic Tafel lines (beta(c) and beta(a)) are approximately constant and independent of the inhibitor concentration. The adsorption of these compounds on C-steel surface has been found to obey the Frumkin's adsorption isotherm. The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitors.