The invention provides a crystalline naphthalene-1,5-disulfonic acid salt of 2-[(S)-1-(8-methylaminooctyl)pyrrolidin-3-yl]-2,2-diphenylacetamide or a solvate thereof. This invention also provides pharmaceutical compositions comprising the salt or prepared using the salt; processes and intermediates for preparing the salt; and methods of using the salt to treat a pulmonary disorder.