A process for preparing (3R)-3-[2-Hydroxy(di-2-thienyl)acetoxy]-1-(3-phenoxypropyl)-1-azoniabicyclo[2.2.2]octane bromide (aclidinium bromide) comprises reacting 2-hydroxy-2,2-dithien-2-ylacetic acid 1-azabicyclo[2.2.2]oct-3(R) yl methyl ester and 3-phenoxypropyl bromide, wherein the reaction takes place in a solvent or mixture of solvents selected from the group of amides and/or the group of solvents with a sulfoxide group. Also provided is a crystalline aclidinium bromide characterized by a powder XRPD pattern having peaks at 7.7±0.2° 2θ, 10.4±0.2° 2θ, 13.2±0.2° 2θ, 13.8±0.2° 2θ, 19.9±0.2° 2θ, 20.3±0.2° 2θ, 20.8±0.2° 2θ, 24.2±0.2° 2θ, 25.7±0.2° 2θ, 26.1±0.2° 2θ, 29.2±0.2° 2θ, 30.8±0.2° 2θ. A pharmaceutical composition comprises aclidinium bromide according to the invention and a pharmaceutically acceptable excipient.