An outline of the structure of the catalyser component Bi2Mo2O9 has been determined from X-ray powder diffractometer diagrams. The space group is P2 l n (= P2 l c) with cell constants: a = 11.946 (2) A ̊, b = 10.795 (2) A ̊, c = 11.876 (2) A ̊ and β = 90.15 (2)°. There are eight formula units per cell. The positions of the metal-ions could directly be derived from the intensities of the strongest reflexions. With difference terms calculated from 19 strong reflexions a Δ F-synthesis was calculated, which revealed the approximate positions of the O2--ions. From packing considerations it was apparent that of 9 02--ions one is only coordinated by Bi3+ and that Mo6+ must be tetrahedrally surrounded by 02-. These tetrahedra may be strongly distorted. The structure is not related to the scheelite-structure as has been assumed by some authors.