In the past few years, there has been a growing popular backlash against international institutions. Examples include the 2015 Greek bailout referendum, the 2016 Brexit referendum, or the 2016 election of a US President seemingly determined to withdraw US support from various international treaties. The implications of these mass-based disintegration efforts reach far beyond the countries in which they originate. First, the disintegration process is shaped by how remaining member states respond to one member’s bid to unilaterally change or terminate the terms of an existing international agreement. Second, mass-based disintegration bids pose considerable political contagion risks by encouraging disintegrative tendencies in other countries. Unfortunately, our theoretical tools to understand such international disintegration processes are underdeveloped. DISINTEGRATION therefore conducts a broad, systematic, and comparative inquiry into the mass politics of disintegration that pays particular attention to reactions in the remaining member states. DISINTEGRATION explores three main research questions: 1) PUBLIC OPINION. When and how does one country’s mass-based disintegration experience encourage or deter demands for disintegration in other countries? 2) DOMESTIC DISCOURSE. How are the contagion effects of mass-based disintegration transmitted through domestic elites and domestic discourse? 3) DISINTEGRATION NEGOTIATIONS. How do the remaining member states respond to one member state’s mass-based disintegration bid? DISINTEGRATION’s main objective is to generate a theory of mass-based disintegration on the basis of a broad, systematic, and comparative inquiry. Rather than examining why voters opt for disintegration, it takes one state’s mass-based withdrawal decision as the starting point and examines how it reverberates across the remaining member states. The project pays particular attention to the role of contagion effects, the dilemmas and incentives they generate for policymakers, and the dynamics they produce in the international arena. Empirically, DISINTEGRATION exploits the unique research opportunity that two ongoing mass-based disintegration processes offer: the Brexit process and an upcoming Swiss referendum aimed at terminating a Swiss-EU bilateral treaty. It undertakes large-scale multi-method data collection that exploits the research opportunities offered by two ongoing mass-based disintegration processes: the Brexit negotiations and an upcoming Swiss referendum aimed at terminating a Swiss-EU bilateral treaty. In terms of methodology, it combines public opinion research, text-as-data-methods, and comparative case studies. DISINTEGRATION’s main objective is to develop a much-needed theory of mass-based disintegration that helps us understand the transnational dynamics that unfold between governments, political elites and the mass public when one member state attempts to unilaterally withdraw from an international agreement on the basis of widespread popular support.