One of the great achievements of the Asser Instituut, under Voskuil's inspiration, has been the study and documentation of international arbitration. It may be appropriate therefore to present here a chapter on one aspect of arbitration on which documentation is scarce and about which little has been written.
The question we shall consider is the familiar one of applicable law: what is the law governing foreign investments made under an investment treaty? By what law should arbitrators be guided when they deal with international investment disputes? Some of the investment treaties concluded to promote and protect international investments answer these questions, which of course are essentially questions of private international law. The treaty rules on applicable law often refer arbitrators to rules of international law as well as to the national law of the host state (including its private international law). Thus, we are concerned with mat grey zone between public and private international law where it is often difficult to find one's way or to ascertain what rules should be applied: those of municipal law (of which private international law forms part) or those of (public) international law. It is widely held by international lawyers that rules of public international law take precedence over national law and therefore over private international law, whenever a conflict arises between them. This question, however, is controversial; U.S. doctrine, for instance, holds that national (U.S.) laws take precedence over international law if they are expressed in sufficiently clear terms.