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Application of inhibitors is an established and cost-effective method to mitigate internal corrosion of mild steel pipelines in the oil and gas industry. Conventionally, surfactant-type organic inhibitors are frequently applied based on their critical micelle concentration (CMC) values and their adsorption to mild steel evaluated based on laboratory tests that show a reduction in corrosion rate. In this work, the relationship between reduction in corrosion rate, CMC and inhibitor surface saturation concentration on mild steel was studied using model quaternary ammonium inhibitors with different alkyl tail lengths. The quaternary ammonium model compounds were synthesized in-house and characterized by 1H-NMR before their use. Their CMCs were determined using surface tension measurements. Results showed that, although the CMC value and surface saturation concentration were the same for two of the inhibitors tested, there was no relationship observed between measured CMC values, surface saturation concentrations, and the calculated corrosion efficiencies for the five model inhibitor compounds tested. Consequently, using CMC values as a measurement for injection of inhibitors might not be considered as a reliable factor.