The jurisdiction of an international arbitrator or an arbitral institution which applies International Investment Law requires the prior existence of a foreign investment. This study analyzes the difficulties of verifying the existence of an investment in front of the International Centre for Settlement of Investment Disputes (ICSID) and suggests some ways to overcome them. The importance of the Washington Convention of March 18,1965 explains the special attention paid to ICSID. The author takes into account the various approaches proposed by the doctrine and jurisprudence and their foundations. The approach then suggested in this article is based on a dissociation of the conventional sources of ICSID arbitrator's jurisdiction. The author notes that each legal instrument has a specific and limited scope. He then proposes that the controlling operations of the investment condition be based on the separation of the normative sources which constitute the legal basis of the arbitrator's jurisdiction, most often a bilateral treaty on investment and the Washington Convention.