In this present study, 1-butyl-3-methylimidazolium chloride ([C(4)MIM]Cl), 1-octyl-3-methylimidazolium chloride ([C(8)MIM]Cl), and 1-decyl-3-methylimidazolium chloride ([C(10)MIM]Cl) were adopted as mobile phase additives in the high performance liquid chromatography (HPLC) to simultaneously separate phenoxy acid herbicides and phenols at neutral pH. It was found that by using 20 mM of [C(4)MIM]Cl, baseline separation and good chromatograms for all the acid compounds were obtained on a normal reversed-phase C(18) column. The retention time of the target acid compounds shortened with the increase of the alkyl chain length and the concentrations of ionic liquids, probably due to the delocalization of the positive charge on the imidazolium cation, the repulsion between chlorine ions of ionic liquids and the acid compounds, as well as the stereo-hindrance effect. The mechanism with ionic liquids as mobile additives for the separation of acid compounds was discussed.