The isolation and recovery of naphthenic acid from highly acidic model oil was performed using hydroxide-based ionic liquids. An extremely low ionic liquid/oil ratio was used to completely deacidify the model oil. Tetraalkylammonium and tetraalkylphosphonium hydroxide ionic liquids were employed in this study. Tetraalkylammonium-based ionic liquids are more efficient than tetraalkylphosphonium-based ionic liquids. The recyclability of the ionic liquids for the extraction of naphthenic acid was also studied. The regeneration of the extracted naphthenic acid was achieved. © 2014 American Chemical Society.