Here we present a report on synthesis and characterization of tetravalent metal pyrophosphate (TMP) and alkali carbonate (A(2)CO(3); A = Li and/or Na) composites. The TMP-carbonate composites are prepared by mixing indium-doped tin pyrophosphate or yttrium-doped zirconium pyrophosphate with Li2CO3 or an eutectic mixture of Li2CO3-Na2CO3 in different wt.% ratios. The phase composition, microstructure and electrical conductivity of the sintered specimen are analyzed. In addition, the effect of different TMP and A(2)CO(3) phases is investigated. A maximum ionic conductivity of 5.5 x 10(-2) S cm(-I) at 630 degrees C is observed in this study with a Sn(o.9)I(no.l)P(2)0(7)-Li2CO3 composite. Based on the literature data, TMP-carbonate composites can be considered to be primarily a proton and oxygen -ion co-ionic conductor and, therefore, have strong potential as electrolytes in fuel cells in 500-700 degrees C range. (C) 2017 Elsevier B.V. All rights reserved.