The synergistic effects of some ions such as KI, KCl and KBr on the corrosion inhibition of zinc in 0.4 M HCl by tetrahydrocarbazole derivatives compounds has been investigated using weight loss measurements. Potentiodynamic anodic polarization technique was used to measure the pitting corrosion potential and Quantum Chemical studies (Computational Study) were investigated. Results obtained show that these compounds are good inhibitors and their inhibition efficiencies (IE%) increase with the increase of inhibitor concentration. ..