The inhibition performances of some selected azine and thiazine dyes, namely, Neutral Red (NR), Azure A Eosinate (AAE), Toluidine Blue (TB), phenosafranin (PS), and Rhodanile Blue (RB), on mild steel corrosion in hydrochloric acid solution was studied the using electrochemical impedance spectroscopy (EIS) and Tafel polarization techniques. Quantum chemical calculations based on the density functional theory (DFT) and semiempirical (PM3) methods were used to investigate the reactivities and selectivities of the studied cationic dyes. The effects of inhibitor concentration on the inhibition efficiency have been studied. Inhibition efficiency increased with increase in concentration of all the studied cationic dyes within the concentration range 100-500 ppm. Potentiodynamic studies revealed that all the inhibitors are of mixed type. The results obtained from the EIS studies showed good agreement with the results from potentiodynamic polarization techniques. The quantitative structure-activity relationship (QSAR) approach was also used to correlate the quantum chemical parameters with the experimentally determined inhibition efficiencies. The results show that thiazine dyes are better corrosion inhibitors than azine dyes; however, when azines contain more electron donor centers than thiazines, they are preferred as corrosion inhibitors to thiazine. Hydrogen bonding could be one of the possible physisorption mechanisms for the adsorption of the selected dyes onto the metal surface because of the many hydrogen bond donor centers in the studied compounds. QSAR results show good correlations between a number of quantum chemical parameters and the determined inhibition efficiency.