Crystals of the title compound are triclinic, space group P, a= 12·054(13), b= 7·695(8), c= 10·183(11)Å, α= 103·9(1), β= 105·8(1), γ= 96·5(1)°, Z= 1. The structure was determined from diffractometer X-ray data by Patterson and Fourier methods and refined by full-matrix least-squares techniques to R 0·063 for 1366 independent reflections. The compound has imposed symmetry. The two molybdenum atoms in the asymmetric unit have different environments although both are formally seven-co-ordinate, being bonded to each other [2·669(2)Å], to one chlorine atom [2·395(5), 2·367(6)Å] and five oxygen atoms. The metal–metal bond has little effect on the geometries of the co-ordination polyhedra of the molybdenum atoms which are best described as distorted octahedra. These four octahedra in the tetramer are bridged through edges. Two oxide oxygens form terminal bonds [1·617(15) and 1·634(16)Å], the third bridging three molybdenum atoms [1·993(9), 1·962(1 2), and 2·237(1 3)Å]; one propoxide oxygen is terminal [2·134(13)Å], and the other two bridge pairs of molybdenum atoms [1·981 (11), 1·985(14), 2·048(13), and 2·182(14)Å]. Variations in Mo–O bond lengths are explained in terms of differing bond types (and orders). The propoxide carbon atoms have high thermal motion and some are disordered.