In recent years, Russia has invested significant assets in unique scientific facility of the “Megascience” class that are being built or are already operating on the territory of foreign countries, mainly member states of the European Union: the International Thermonuclear Research Reactor-ITER (France), the European X-ray Free Electron Laser-European XFEL, the Large Hadron Collider (Switzerland and France), etc.How reliable and safe are such investments in the context of the sanctions policy of the West, including the EU, against our country? To what extent are they protected by the principle of immunity of states and international organizations, which is generally recognized, but is not interpreted and applied in different legal systems? The paper considers these issues in the context of the development of the judicial practice of the supranational institution of the judicial power of the EU, namely the Court of Justice of the European Union and the concept of relative immunity (immunité relative) formulated herein.Having conducted a comparative legal review of the current state of the sources of law and doctrine on the issues of immunity of states and international organizations, the author analyses and evaluates the decisions of the EU Court of Justice and the legal positions of its attorneys General: — Mahamdia v. Algeria, 2012: for the first time ECJ formulates the concept of relative immunity in relation to states;— "Rina" and "Suprim" cases, 2020: EU Court clarifies the interpretation of the concept of acta iure imperii (acts of public authority), in respect of which states retain immunity in the EU, and extends its concept of relative immunity to international intergovernmental organizations.The final section deals with legal issues that yet to get a clear answer in the practice of the EU Court of Justice. In this regard, the author highlights possible directions of its evolution, and studies other recent decisions of the EU Court of Justice that may affect Russia’s national interests in the context of cooperation with EU member states in the scientific and technical sphere, including megascience, and in other areas.