The effect of 2-oleyl-1-oleylamidoethyl imidazoline ammonium methylsulfate (ODD) and halide ions on corrosion inhibition of mild steel in HCl solution has been studied by experimental and molecular dynamics simulation methods. Synergistic effects were observed between ODD and the halides, and the inhibition efficiency was found to follow the trend ODD-Cl- < ODD-Br- < ODD-I-. In molecular dynamics simulation, the analysis of fractional free volume and diffusion coefficient showed that the synergistic effect increased in the order ODD-Cl- < ODD-Br- < ODD-I-. The molecular dynamics simulation analysis agreed with the experimental results. (C) 2012 Elsevier B.V. All rights reserved.