A new salt-co-crystal of tiotropium fumarate with fumaric acid has been discovered, and found to be the most stable solid form of tiotropium fumarate. This type of structure consists of matched cations and anions (a salt) together with a nonionized free acid moiety as the co-former (co-crystal), and is unique amongst the large number of tiotropium salts that have been prepared and characterized. The stoichiometry cation/anion/co-former of 2:1:1 corresponds to a simple polymorph of the 1:1 salt, and its identity as a co-crystal has been established by single-crystal X-ray diffraction with some corroborating evidence from the Raman and infrared spectra. A detailed investigation of the bonding and geometry of the three crystalline forms of the fumarate indicates that the hydrogen bonding motifs are very similar, and that conformational differences arising from the packing of the two thiophene rings into the crystal structure is probably important in determining their relative stabilities. A comparison with the structures of other tiotropium salts indicates that there is a correlation of the dihedral angle between the two tiotropium thiophene rings with the stability of the crystal forms.