Alkaline earth substituted UO2 (U1-xMxO2 +/-delta ; M=Mg, Ca, Sr; 0.1 <= x <= 0.525) with fluorite structure was synthesized in reducing atmosphere. Structure and conductivity properties of U1-xMxO2 +/-delta fluorites were investigated for possible application in solid oxide fuel cells (SOFC). At room temperature and ambient atmosphere the materials are stable; however they decompose at an oxygen partial pressure po(2) > 10(-4) aim and temperatures higher than 600 degrees C. The total conductivity measured for the best conducting U1-xMXO2 +/-delta material with M=Ca and x=0.177 is as high as 3 S/cm at po(2) < 10(-4) aim at 600 degrees C. The relatively low ionic transference number (t(i)similar to 0.02) is disadvantageous for potential use as electrolyte material for SOFC applications. The high conductivity and possible depolarization effects suggest potential use as anode materials in SOFC. (c) 2005 Elsevier B.V. All rights reserved.