Abstract <p>The effect of formic acid as a mobile phase additive on the retention time of five compounds (sodium nitrite, potassium iodide, acetone, uracil, oxalic acid) selected as dead time markers was studied in reversed-phase high-performance liquid chromatography (RP HPLC). All the studied compounds were shown to have weak retention on the surface of octadecylsilica gel under neutral conditions. However, the introduction of an acidic additive to the mobile phase leads to an increase in the retention of compounds giving an alkaline reaction. This effect was particularly strong for sodium nitrite. The lowest retention time was observed for oxalic acid, and it can be recommended as a dead time marker for the analysis of weakly polar and nonpolar compounds in neutral and acidic media.</p>