The crystal structure and transport properties of (Sm2 - xZrx)Zr2O(7 + x/2) (x= 0; 02; 032; 0.39; 0.48; 0.67; 0.78; 0.96; 1.14; 1.27) solid solutions have been investigated by X-ray techniques and impedance spectroscopy, respectively. The excess oxygen content of the composition with x = 02 has been determined by thermal analysis and mass spectrometry in a reducing atmosphere. The SmZrO system includes a two-phase (fluorite + pyrochlore) region for the (Sm2 - xZrx)Zr2O7 (+ x/2) (0.48 <= x < 0.96) solid solutions. The interstitial oxide ion conductivity of the (Sm2 - xZrx)Zr2O7 + x/2 (0.2 <= x < 0.48), 3 x 10(-3) S/cm at 750 degrees C, is comparable to the vacancy-mediated conductivity of undoped Sm2Zr2O7. The bulk conductivity of the interstitial oxide ion conductors (Sm2 - xZrx)Zr2O7 + x/2 (0.2 <= x < 0.48) was shown to vary little in a wide range of Sm2O3 concentrations in contrast to the vacancy mediated oxide ion conductors Sm-2(Zr2 - xSmx)O7 - delta (0 <= x < 029). (C) 2014 Elsevier B.V. All rights reserved.