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The effect of sodium thiosulfate (Na2S2O3) concentrations (0.01, 0.1 and 1 M) on carbon steel corrosion behavior in 3.75 M ammonium chloride (NH4Cl) solution was investigated. The potentiodynamic polarization results show that the icorr increases from 8.7 × 10−6 A cm−2 to 7.2 × 10−4 A cm−2 when thiosulfate (S2O32−) concentration increases from 0 to 1 M. Electrochemical impedance spectroscopy measurements also mimic the same trend. However, it is also observed that the icorr value starts decreasing when S2O32− concentration equals or exceeds the NH4Cl concentration (Cl−:S2O32− ≥ 1:1) in the electrolyte solution. The polarization behavior and field-emission scanning electron microscopy images reveal that the carbon steel undergoes general corrosion at lower concentrations (0.01 M S2O32− and 0.1 M S2O32−) and, both pitting and general corrosion at higher concentration (1 M S2O32−). Based on the experimental results, the mechanistic reaction pathway, which involves two dissolution paths for carbon steel dissolution is suggested. Keywords: Carbon steel, Ammonium chloride, Sodium thiosulfate, Electrochemical techniques, Corrosion mechanism