The effect of two pyrazole-type organic compounds, namely ethyl 5,5'-dimethyl-1'H-1,3'-bipyrazole-3 carboxylate (P1) and 3,5,5'-trimethyl-1'H-1,3'-bipyrazole (P2) on the corrosion behaviour of steel in 1 M hydrochloric acid (HCl) solution is investigated at 308 K by weight loss measurements, potentiodynamic polarisation and impedance spectroscopy (EIS) methods. The inhibition efficiencies obtained from cathodic Tafel plots, gravimetric and EIS methods are in good agreement. Results obtained show that the compound P2 is the best inhibitor and its efficiency reaches 84% at 10(-3) M. Potentiodynamic polarisation studies show that pyrazolic derivatives are cathodic-type inhibitors and these compounds act on the cathodic reaction without changing the mechanism of the hydrogen evolution reaction. The inhibition efficiency of P2 is temperature-dependent in the range from 308 to 353 K and the associated activation energy has been determined. P2 adsorbs on the steel surface according to Langmuir adsorption model. The calculation of the total partial charge of inhibitor atoms is computed. (c) 2005 Elsevier B.V. All rights reserved.