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