The inhibitive performance of novel synthesised water-soluble triblock copolymers - 2-(diethylamino)ethyl methacrylate-block-2(dimethylamino)ethyl methacrylate-block-2-(N-morpholino)ethyl metbacrylate [PDEA-PDMA-PMEMA] and 2-(diisopropylamino)ethyl methacrylate-block-2-(dimethylamino)ethyl methacrylate-block-2-(N-morpholino)ethyl methacrylate [PDPA-PDMA-PMEMA] of two different molecular weight - on the corrosion behaviour of mild steel in 0.5 M HCl were investigated using potentiodynamic polarisation, electrochemical impedance spectroscopy and linear polarisation methods. Polarisation curves indicate that all studied copolymers were acting as mixed type inhibitors. All measurements show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that inhibitive actions of inhibitors were mainly due to adsorption on steel surface. Adsorption of these inhibitors follows Langmuir adsorption isotherm. Thermodynamic parameters of adsorption (K-ads, Delta G(ads)) of studied triblock copolymers were calculated using Langmuir adsorption isotherm. The variation in inhibitive efficiency mainly depends on the type and the nature of the substituents present in the inhibitor molecule and also depends on the molecular weight of the inhibitors. The correlation between the inhibition efficiencies of studied triblock copolymers and their molecular structures has been investigated using quantum chemical parameters obtained by PM3 semi-empirical SCF-MO methods. These results indicate that adsorption of studied triblock copolymers depends on the charge density of adsorption centres and dipole moments. (c) 2007 Elsevier B.V. All rights reserved.