Electrical conductivity and ion transfer numbers in complex oxides of Me6M2O11(Me:Sr, Ba; M: Nb, Ta) type and solid solutions based on them are investigated. The high mobility of oxygen ions is shown to be due to oxygen structural vacancies V, and the decrease in ion conductivity in Me6M2O11–Me4M2O9 solid solutions is caused by the appearance of M-type substitutional defects and simultaneous filling of oxygen structural vacancies. The model of solid solution formation is corroborated by picnometric and vibrational spectroscopic methods. Self-diffusion coefficients of cations in niobate ceramics Sr5Nb4O15, Sr2Nb2O7, SrNb2O6 and Ba0,4Sr0,6Nb2O6 monocrystals are measured. The proximity of Nb5+ and Me2+ ion mobilities is established and attributed to filling up of V by Nb5+ ions and Nb and V defect formation. [Russian Text Ignored].