Abstract: An antiinflammatory steroid exhibiting a tendency to crystal growth in suspension in aerosol propellants is contacted with a halogenated hydrocarbon to form a crystalline solvate, which, after removal of some or all of the propellant from the crystals, is reduced to a particle size permitting inhalation into the human bronchial system when dispersed as an aerosol, the micronised crystalline solvate not thereafter exhibiting crystal growth in the aerosol propellant. The solvate is a new composition of matter which has been characterized by I.R. spectra and the aerosol formulation prepared therefrom has valuable therapeutic properties, particularly in the treatment of asthma.