How far and under what conditions may experimentalist governance be an efficient and legitimate means of responding to diversity among EU member states, in comparison to both conventional uniform regulation and differentiated integration? By comparing two major domains where the challenge of integrating national diversity has arisen prominently, electricity and banking, we find that under conditions of high interdependence and high uncertainty, diachronic experimentalism may be a necessary condition for synchronic uniformity. Uniform rules can be accepted as efficient and legitimate by member states, provided that they are regularly revised based on implementation experience through deliberative review processes in which national officials themselves participate. Our findings on EU banking regulation further suggest that experimentalist governance and differentiated integration may also be complementary, but asymmetrically so, in that the latter depends on the former to accommodate diversity within and across member states, but not vice versa.