Although liberal intergovernmentalism claims that economic interest groups shape national preferences towards integration, while neofunctionalism assumes that these groups support integration for its expected economic benefits, these approaches cannot account for variation in EU integration across policy areas. We employ an analytical framework to explain divergent firm preferences towards integration in the EU-wide internal energy market. Building on Weber and Hallerberg’s (2001) specification of transaction costs and external (competitive) threat as independent variables in their model of divergence in firm preferences towards ‘binding’ EU rules, our analysis incorporates domestic market structure and firms’ international relationships as intervening (contextual) variables. Testing our argument in four cases - Germany, Italy, France and the UK - confirms that distinct national institutions promote divergent attitudes towards the internal energy market because domestic market structures and firms’ international settings respond to transaction costs and external threat in this market within the context of member states’ traditional local models of capitalism. In relation to theories of European integration, this study underscores the importance of varieties of capitalism in preference formation vis-à-vis integration, offering additional insights into the conditions under which national institutions have been influential in response to common external pressures in the energy market.