Abstract: Nowadays, when a matter of general interest is raised in arbitrations between a State and an investor, civil society claims the right of being heard by intervening in the proceeding as amicus curiae. At an interstate level, the appellate body of the World Trade Organization (WTO) has already acknowledged, in various occasions, its discretionary power to accept memorials submitted by amicus curiae, stating the legal grounds for a favorable acceptance that should be reserved to it. Based on such precedents, two arbitral tribunals constituted under Chapter 11 of the Nafta have adapted this solution to mixed arbitration involving foreign investment. Inspired by applicable procedural rules, but distinguishing between the status of a party and that of an amicus curiae, arbitrators admitted the possibility of non-governmental organizations for protection of environment, as well as syndicated associations, presenting in writing their arguments relating to the merits of the dispute, although denying their participation in the hearings and access to the records of the proceeding. While disapproved by interested persons - and especially by private investors -, the opening to civil society, guided by a concern with transparency and respect for public interest, establishes a new stage towards marginalization of the parties' consent within an arbitral proceeding. While the wish of arbitral tribunals in admitting memorials submitted by amicus curiae seems to be unanimous, the modalities and potential impact of such admission are still unknown.