Peacekeeping operations conducted by international organizations raise difficult questions of international responsibility. In principle, breaches of international law committed by national contingents serving on such operations may be attributed either to the international organization leading the operation or to the State to which the personnel implicated in the wrongful conduct belongs. The ARIO suggests a seemingly simple solution to this dilemma: wrongful conduct should be attributed to the party exercising effective control over that conduct. The present note argues that this solution is misguided. It deliberately ignores the legal and institutional status of national contingents, does not reflect consistent international practice and may not serve the best interests of potential claimants. In the case of peacekeeping operations incorporated into the institutional structure of an international organization, a more appropriate solution to the dilemma of multiple attribution is to proceed on the basis of a rebuttable presumption that the wrongful acts committed by national contingents are attributable to the international organization and not to their contributing State.