The concept of ‘facultative mixity’ as first coined by Allan Rosas3 has sparked a much heated
debate.4 Is it a matter of political expediency in the EU Council to decide on the mixed nature,
or not, of a given agreement in so far as it falls within shared competence of the EU and its
Member States? Considered as such, this concept is offset against ‘obligatory’ or ‘compulsory
mixity’ which would then arise only where the Member States retain an exclusive competence
for part of the agreement. It is apparent that the concepts of facultative and obligatory mixity
so understood both rest on the premise that the mixed nature of an agreement is to be
determined solely on the basis of the division of competence under the EU Treaties. The crucial
exercise then lies in the correct appraisal of the ‘partial nature’ of the conferral of competence
under the EU Treaties which, of itself, may prove to be a difficult exercise not least in a post-
Lisbon setting.
