There is support in the investmel1l treaty cases for the proposition that the lawfulness of therninvestment is a condition precedent for the conferral of adjudicative power upon tribunals orrnfor the availability of substantive protections at the merit phase.rnThis plea of legality isrnjustified by reference to arguments either: (i) an express provision of the treaty requires thatrnlawjii/ness of the investment; or (ii) the notion that investment treaties should give effect torngeneral principles of laws such as good faith or the maxim that a ciaimal1l should not be ablernto profit from its own wrongs. However, in effect, the decision on when to address the plea ofrnillegal investmel1l has proved to have shown potentially significal1l consequences for thernrespondent, the claimant and for the general outcomes of investment arbitrations. In this vein,rnmany legal scholars who have studied the trend of this discourse seemed to have supportedrnthe idea that when an express provision of the applicable treaty requires the lawjitlness of therninvesll17ent, the tribunal must consider the plea of illegal investment as jurisdictional question.rnConversely, where there is no an express provision of the applicable treaty requires thernlawfii/ness of the investment, tribunals must consider the mailer as regards the merit of therndispute either to affect the substantive protection or the amount of compensation due. Thisrnthesis, however, defended that such conclusions are not amenable to be a consistent broad-rnspectrum and unified interpretation as can be evidenced by examination of more inclusivernauthoritative international investment cases, which instead proposed that adjudication to bemade by giving significant emphasis to the specificity of a given qualification in the lIA,rnand/or of the circumstances of the aClllal case.