Potentiodynamic polarisation, electrochemical impedance spectroscopy and weight loss studies were carried out on the inhibition of carbon steel in I M hydrochloric acid solution by three bipyrazolic compounds. The examined inhibitors are 5,5-dipropyl-I H, 1'H-[3,3']bipyrazolyl (Bpl), 5,5diphenyl-1H,1'H-[3,3']bipyrazolyl (Bp2) and 5,5'-bis-(4-chloro-phenyl)-1H,1'H-[3,3']bipyrazolyl (Bp3). All measurements show that inhibition efficiency increases with increase in inhibitor concentration and the variation in inhibitive efficiency mainly depends on the type and nature of the substitutents present in the inhibitor molecule to attain 98% for chloro derivative (Bp3) at 10(-3) M. This reveals that inhibitive actions of bipyrazolic compounds were mainly due to adsorption on steel surface. Adsorption of these inhibitors follows Langmuir adsorption isotherm. Electrochemical impedance spectroscopy (EIS) measurements showed that the dissolution process of steel occurred under activation control. The curves of polarisation indicated that the examined inhibitors act primarily as cathodic inhibitors. From the adsorption isotherm, thermodynamic data for the adsorption process (K, Delta G(ads)(o)) are calculated and discussed. Some quantum parameters (highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy and total partial charge of inhibitors atoms) are computed. (c) 2007 Elsevier B.V. All rights reserved.