In recent years, critics have questioned the legitimacy of international investment law, particularly investor-State arbitration on the grounds, amongst others, that confidentiality and lack of transparency in arbitral proceedings pose a threat to the basic principles of public law and democracy. In response, minimal transparency measures have been introduced by States, regional international economic organizations and the International Centre for the Settlement of Investment Disputes (ICSID) over the last two decades. More recently, the Transparency Rules and the Mauritius Convention were introduced by the United Nations Commission on International Trade Law (UNCITRAL) for a more far-reaching impact. These instruments have been widely applauded as the much awaited solution for entrenching transparency and enhancing the legitimacy of treaty-based investor-State arbitration. But will they really establish transparency in investor-State arbitration considering the opt-out provisions in Article 1(1) of the Transparency Rules and Article 3(1) of the Mauritius Convention? In attempting this question, the article examined the concept of treaty based investor-State arbitration, its public character and the possible effect the opt-out provisions could have on the quest for transparency. It posited that a mechanism that allows parties – States and foreign investors – a choice whether or not to apply these instruments in a given arbitration will impede the attainment of the objective of entrenching transparency in investor-State arbitration.