One of the most urgent and widely discussed problems of international organisations is their need for democratic legitimacy. In this article, a novel approach is developed for the study of democracy in international context. This approach is borrowed from sociological theory and focuses on Shmuel Eisenstadt's and Niklas Luhmann's views on social differentiation and illustrated through the current debate over the democratic deficits of international arbitration. The proposed differentiation theory provides a dynamic and abstract explanatory framework which apart from offering a conceptual frame of reference for the judicialisation of international relations, makes the concept of democracy more specific and precise. Moreover, in contrast to other theories, it explains the claims for democracy arising out of both differentiation and dedifferentiation processes. Ultimately, this article argues that the claim for democracy is inadequate when applied to international arbitration and obscures its need for other forms of legitimisation.