The feasibility of doping Ga3+ into the Aurivillius family of layered perovskites has been re-assessed in the light of the previously reported high oxide ion conductivity in these phases. Careful X-ray powder diffraction (XRD) studies reveal that the Ga3+ solubility in both Bi2Sr2Nb2TiO12 and BaBi4Ti4O15 is very limited, and the previously reported `oxygen-deficient' phases `Bi2Sr2Nb2GaO11.5' and `BaBi4Ti3GaO14.5' are, in fact, mixtures, both containing a significant amount of a beta-Bi2O3-related phase. Since this phase is known to transform to the highly conducting delta-Bi2O3 phase at elevated temperature, reports of oxide ion conductivity in these `oxygen-deficient' Aurivillius phases should be treated with caution. This work highlights the importance of careful structural characterisation of complex materials in polycrystalline form using a reliable structural model and quantitative profile-fitting methods, rather than simply `indexing' based on peak position data alone. (C) 2003 Elsevier Science B.V. All rights reserved.