A new surfactant, dioleyl phosphoric acid (DOLPA), has been applied to the extraction of hemoglobin using reversed micelles. The reversed micelles formed by DOLPA can easily extract hemoglobin from aqueous to reversed micellar solutions. DOLPA is the first surfactant to extract hemoglobin completely without using any cosurfactants. On the basis of the difference between DOLPA and AOT reversed micelles in the forward extraction behavior of hemoglobin, the nature of the interfacial complex that would be formed between surfactants and hemoglobin at the oil-water interface was found to be the dominant factor in determining the extraction efficiency of hemoglobin by reversed micelles. In addition, back-transfer studies of hemoglobin from the DOLPA reversed micelles were also carried out by the phase transfer method. It was found that hemoglobin, once dissolved into the DOLPA reversed micelles, is not transferred to a fresh aqueous solution even when the conditions are adjusted to not allow the forward transfer of hemoglobin. However, the addition of several kinds of alcohol drastically improved the yield in the back-transfer of hemoglobin. The efficiency in the back-transfer of hemoglobin strongly depends on the aqueous conditions that are in contact with the reversed micelles, such as pH, ionic strength, and alcohol concentration. A pH higher than the pi of hemoglobin, a salt concentration lower than that of the water pool, and the proper concentration of alcohol are required for the recovery aqueous phase to ensure the back-transfer of hemoglobin from the DOLPA reversed micelles.