Ukraine, upon giving up the nuclear arsenal left on its territory by the USSR, entered in 1994 into a Memorandum on Security Assurances with the United Kingdom, United States and Russian Federation (Budapest Memorandum). Since the crisis began between the Russian Federation and Ukraine in February 2014, a number of States have invoked the Budapest Memorandum. Unclear, however, is whether this instrument constituted legal obligations among its Parties or, instead, is a political declaration having no legal effect. The distinction between political instruments and legal instruments is a recurring question in inter-State relations and claims practice. The present article considers the Budapest Memorandum in light of the question of general legal interest – namely, how do we distinguish between the legal and the political instrument?