Corrosion inhibition efficiencies of heterocyclic, unsaturated (aromatic and nonaromatic) compounds (pyrimidines, benzothiazole derivatives, amino acids containing an aromatic part, pyridines, and quinolines) were correlated with quantum chemical indices of the respective molecules. Inhibition efficiencies were determined in acidic solutions containing 0.001 M or 0.01 M of the inhibitor. Quantum chemical calculations were made by using the Hückel method. The difference (Δ) between energy of the highest occupied and the lowest unoccupied molecular orbital was related to the inhibition efficiency (E) of the molecules. At values Δ < 1.3 beta, efficiency increased with increasing values of Δ. At values Δ > 1.3 beta, efficiency (in 0.001 M concentration) tended to decrease with increasing values of Δ. The optimal value of index Δ may have been lower in solutions containing 0.01 M of the inhibitor. Results indicated that Δ alone is insufficient to account for all variations in the experimental inhibition efficiency.