An aerosol formulation containing a particulate active substance having a mass median aerodynamic diameter (MMAD) of less than 10 µm, suspended in a nonsolvent hydrofluorocarbon fluid vehicle at a concentration of 0.2 to 5% w/v, wherein the fluid vehicle is 1,1,1,2-tetrafluoroethane (HFA 134a) or 1,1,1,2,3,3,3-heptafluoropropane (HFA 227ea) and which exhibits a flocculation volume of 35% or greater 30 seconds after mixing of the active substance and vehicle. The particulate active substance may be a pharmaceutically active substance and the formulation may be for use in inhalation therapy, in particular in a metered dose inhaler.
