1-[4-acetyl-2-(4-chlorophenyl)quinoxalin-1(4H)-yl]a cetone (Q2) was examined as a corrosion inhibitor f or carbon steel in 1.0 M HCl by using weight loss measurement s and the quantum chemical studies using density fu nctional theory (DFT) methods. The percentage inhibition eff iciency ( η ), was found to increase with increase of the inhib itor concentration due to the adsorption of the inhibito r molecules on the metal surface. In addition it wa s established the adsorption follows Langmuir adsorption isotherm . Moreover some thermodynamic data were calculated and discussed. Quantum chemical parameters such as high est occupied molecular orbital energy (E HOMO ), lowest unoccupied molecular orbital energy (E LUMO ), energy gap ( ∆ E) and dipole moment (μ ), the softness ( σ ), the fraction of the electrons transferred from the inhibitor to the metal surface ( ∆ N) and the total energy (TE) have been calculated for this compound. It was found that the oretical data support the experimental results.