The structures of the two oxides Bi 3 (MSb 2 )O 11 (M= Al, Ga) have been refined by Rietveld-type analysis of combined powder neutron and X-ray synchrotron diffraction data. The two oxides are isostructural, space group Pn3,a= 9.43323(4) Å forM= Al anda= 9.48980(3) Å forM= Ga, in which theMand Sb atoms are statistically distributed in a 12gsite. The structures can be described as two interpenetrating networks: aM 2/3 Sb 4/3 O 9 array of edge-sharing octahedra and a Bi 2 O 3 array of corner-sharing Bi 8 O 4 stellae quadrangulae. There are two independent Bi sites, both having distorted coordination geometries. The surface composition of both oxides is essentially identical to that of the bulk. Comparison with previous studies suggests that there may be some coupling between the mobility of the Bi ions in these oxides and their electronic properties.