A “mercenary” is a person who takes a direct part in hostilities motivated essentially by the desire for private gain.
⇒ A captured mercenary is not a prisoner of war.
Article 47 of Additional Protocol I spells out six cumulative conditions for a person to be a mercenary. The person must: 

  1. be specially recruited locally or abroad in order to fight in an armed conflict;
  2. take a direct part in the hostilities;
  3. be motivated to take part in the hostilities essentially by the desire for private gain and to be promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
  4. not be a national of a Party to the conflict or a resident of territory controlled by a Party to the conflict;
  5. not be a member of the armed forces of a Party to the conflict; and
  6. not have been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.

The 1989 International Convention against the Recruitment, Use, Financing and Training of Mercenaries extends this definition to persons recruited for the purpose of participating in a concerted act of violence aimed at overthrowing a government or otherwise undermining the constitutional order of a State, or at undermining the territorial integrity of a State.
A national of a neutral State who enlists in the armed forces of a party to the conflict is not a mercenary.
See Private Military and Security Companies (PMSCs);





Suggested readings:
CHESTERMAN Simon & LEHNARDT Chia (eds), From Mercenaries to Market: the Rise and Regulation of Private Military Companies, Oxford, OUP, 2007, 287 pp.
DAVID Éric, Mercenaires et volontaires internationaux en droit des gens, Bruxelles, Éditions de l’Université de Bruxelles, 1978, 460 pp.
FALLAH Katherine, “Corporate Actors: the Legal Status of Mercenaries in Armed Conflict”, in IRRC, Vol. 88, No. 863, September 2006, pp. 599-611.
GREEN Leslie C., “The Status of Mercenaries in International Law”, in IYHR, Vol. 8, 1978, pp. 9-62.
MILLIARD Todd S., “Overcoming Post-Colonial Myopia: A Call to Recognize and Regulate Private Military Companies”, in Military Law Review, Vol. 176, 2003, pp. 1-95.
SCHMITT Michael N., “War, International Law, and Sovereignty: Reevaluating the Rules of the Game in a New Century: Humanitarian Law and Direct Participation in Hostilities by Private Contractors or Civilian Employees”, in ChicagoJournal of International Law, Vol. 5, 2005, pp. 511-546

Further readings:
BOUMEDRA Tahar, “International Regulation of the Use of Mercenaries in Armed Conflicts”, in RDMDG, Vol. 20/1-2, 1981, pp. 35-87.
CASSESE Antonio, “Mercenaries: Lawful Combatants or War Criminals?”, in ZaöRV, Vol. 40, 1980, pp. 1-30.
LIEBLICH Eliav, “The Status of Mercenaries in International Armed Conflict as a Case of Politicization of International Humanitarian Law”, in Bucerius Law Journal, H. 3/2009, December 2009, pp. 115-123.
LILLY Damian, “The Privatization of Peacekeeping: Prospects and Realities”, in Disarmament Forum, Vol. 3, 2000, pp. 53-62.
MACCORMACK Thimothy L.H., “The ‘Sandline Affair’: Papua New Guinea Resorts to Mercenarism to End the Bougainville Conflict”, in YIHL, Vol. 1, 1998, pp. 292-300.
MANDEL Robert, Armies without States: the Privatization of Security, Boulder, London, Rienner Publishers, 2002, 169 pp.
NWOGUGU Edwin I., “Recent Developments in the Law Relating to Mercenaries”, in RDMDG, Vol. 20/1-2, 1981, pp. 9-34.
RAASVELDT Robert, “Accountability Problems for Private Military Companies”, in Humanitäres Völkerrecht, Vol. 3, 2004, pp. 187-189.
SCOVILLE Ryan M., “Toward an Accountability-Based Definition of “Mercenary””, in GeorgetownJournal of International Law, Vol. 37, 2006, pp. 541-581.
SHEARER David, Private Armies and Military Intervention, London, International Institute for Strategic Studies, 1998, 88 pp.
TAULBEE James Larry, “Mercenaries, Private Armies and Security Companies in Contemporary Policy”, in International Politics, Vol. 37/4, 2000, pp. 433-456