The study examined corrosion inhibition of corrosion inhibition of 5-methyl-2H-imidazol-4-carboxaldehyde and 1H-Indole-3-carboxaldehyde on mild steel in acidic medium using weight loss and Density Functional Theory (DFT) methods. DFT calculations were carried out at B3LYP/6-31+G** level of theory in aqueous medium on the molecular structures to describe electronic parameters. The values of thermodynamic parameters such as free energy of adsorption (ΔGºads), adsorption equilibrium constant (Kads), adsorption entropy (ΔSºads), adsorption enthalpy (ΔHºads) and activation energy (Ea) were calculated, analyzed and discussed. The adsorption process on mild steel surface showed that 4-methylimidazol-5-carboxaldehyde and Indole-3-carboxaldehyde obeyed Freundlich and Temkin adsorption isotherms respectively. Also, the molecular parameters associated with inhibition efficiency such as EHOMO, ELUMO, band gap energy (ELUMO- EHOMO), softness (S), electron affinity (EA) and number of electrons transfer were calculated. The higher inhibitory property of 5-methyl-2H-imidazol-4-carboxaldehyde was attributed to the presence of higher number of protonation sites as a result of higher number of nitrogen atoms, increase in number of plane protonated species and higher net charges on the ring atoms. Manuscript profile