Aerosol solution formulations for use in an aerosol inhaler which comprise 8-hydroxy-5-[(1R)-1-hydroxy-2-[[(1R)-2-(4-methoxyphenyl)-1-methylethyl]amino]-ethyl]-2(1H)-quinolinone or a salt thereof, in particular the hydrochloride salt (TA 2005), as an active ingredient, a propellant containing a hydrofluoroalkane, and a cosolvent, stabilized by addition of a specific small amount of a high concentrated phosphoric acid exhibit improved shelf life. The formulation may be optionally contained in a can having part or all of its internal metallic surfaces lined with an inert organic coating.