Mitigation of localized under deposit corrosion (UDC) in upstream oil and gas pipelines is an important research topic for both industry and academia. In a research program to better define the various inhibitor components that provide mitigation of UDC, this initial research investigates the effect of varied ratios of mono-to dinonylphenol phosphate esters (PE) by testing a set of specifically formulated inhibitors. Inhibitors with three monoto di-PE ratios were tested in the presence and absence of 2-mercaptoethanol (ME). Using two 1.25 in (3.18 cm) diameter API 5L X65 pipeline steel samples and 250 μm silica sand, UDC testing was conducted for 28 d in a CO 2 saturated solution at 70°C and 1 bar (100 kPa) total pressure. Analysis has shown that localized corrosion (pit penetration rate) increased for ME-free nonylphenol PE as the concentrations of di-PEs and mono-PEs approached equivalency. The nonylphenol PE inhibitor with a 50:50 mono-to di-PE ratio at 100 ppm concentration failed to protect the surface of the sample under the individual sand grains. Even the base product inhibitor package with no PE provided better mitigation under these test conditions than the 50:50 mono-to dinonylphenol PE. However, it was observed that the addition of ME provided a dramatic improvement in the mitigation of UDC for each mono-to di-PE ratio of the nonylphenol PE tested. From this research, it is seen that the mono-to di-PE ratio is important to consider when developing corrosion inhibitors containing PEs.