Aerosol formulations comprising glycopyrronium bromide in combination with formoterol are useful for the prevention or treatment of chronic obstructive pulmonary disease. The formulation further comprises a HFA propellant, a co-solvent, and an amount of inorganic acid sufficient to stabilize both the glycopyrronium bromide and the formoterol components. Optionally the formulation may further comprise beclometasone dipropionate.
