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Inhibition performance of 3-methyl-1-propargylquinoxalin-2(1H)-one (MPQO) against corrosion of carbon steel in 1.0 M HCl was investigated by weight loss and electrochemical measurements. The inhibition efficiency increased with increasing inhibitor’s concentration, but decreased with the increase in temperature. The results showed that MPQO acts as a mixed type inhibitor in 1.0 M HCl by suppressing simultaneously the cathodic and anodic processes. Adsorption of used inhibitor led to a reduction in the double layer capacitance and an increase in the charge transfer resistance. Thermodynamic analysis indicates that both physisorption and chemisorption probably occur in the adsorption process. The adsorption behaviour of MPQO follows Langmuir’s isotherm.