The solid-state structure of anhydrous (Nr,C,-S) -( -)-scopolamine hydrobromide [ ( -)-hyoscine], an acetylcholine antagonist, has been determined by single crystal X-ray diffraction analysis. ( -) -Scopolamine hydrobromide [an hydrous form] gives crystals belonging to the orthorhombic P2,2,2, space group, and at 298 K: a = 7.348(1), b = 10.482(1), c = 22.867(1) 8, V = 1 761.26(1) A3, Z = 4, R ( F ) = 0.053, and R,(F) = 0.049. The (S)-absolute configuration was determined from the effects of anomalous dispersion of the bromide atom. The N-methyl group exists in an axial configuration similar to that previously described for the hemihydrate. However, in the anhydrous form the tropate residue exhibits a different conformation from that noted for the hemihydrate. The tropate ester moiety in (Nr,C,-S)-( -) -scopolamine hydrobromide [anhydrous form] and (Ns,C,-S) -Nbutylhyoscinium bromide both exhibit very similar crystalline-state conformations, while that in