Complex oxides with the general formula Bi26-2 x Me'2 x Mo10O69- d , Bi26Mo10-2 y Me''2 y O69- d are synthesized by the standard ceramic technology, certified and analyzed by high-temperature X-ray and neutron diffraction techniques. Specific features of the structures of triclinic and monoclinic modifications of complex oxides are determined. It helps interpret a change in the electroconductive properties of the samples as a result of a polymorphic transition: in the monoclinic modification [MoO4-O-MoO4] clusters are detected together with a doubled amount of [(MoO4)-Bi(MoO4)-] n chains oriented along the b axis, which may affect the character of oxygen ion transfer. For Fe-substituted samples selective iron substitution for molybdenum is revealed.