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The anticorrosion performance of two pyrimidine derivatives 2-mercaptopyrimidine (2-MP) and 4-methyl-2-mercaptopyrimidine (4 M−2MP) on Q235 plain carbon steel in CO2-saturated 3 wt% NaCl aqueous solution has been comparatively examined utilizing weight loss methods, electrochemical techniques as well as surface analysis. The results indicate that the two kinds of pyrimidine derivatives serve as mixed-type inhibitors and show excellent and long-lasting inhibitory effect for Q235 steel against general and localized corrosion even at very low concentrations. The inhibition efficiencies increase with a rise in concentration but decrease with temperature rising. Compared with 4 M−2MP inhibitor, the 2-MP exhibits stronger adsorption ability and higher inhibition efficiency due to the existence of two tautomers of thiol and thione forms. The adsorption of both inhibitors over Q235 steel surface can be well fitted by Langmuir adsorption isotherm. The values of standard Gibbs free energy of adsorption ( Δ G a d s 0 ) indicate that both physisorption and chemisorption processes have taken place. Furthermore, thermodynamic parameters are extracted from Langmuir rule to discuss the adsorption mechanism and the XPS analysis is applied to describe the bonding characteristics between inhibitors and steel substrate.