The present invention provides a medicinal aerosol suspension formulation for MDI administration, comprising:a) micronised β2-agonist;b) micronised corticosteroid;c) a sub-therapeutic quantity of a moisture-scavenger excipient; andd) a HFA propellant;wherein (a), (b) and (c) and their respective relative amounts are selected such that they associate to form floccules having a density substantially the same as that of the HFA propellant.