The electrical conductivity of the pyrochlore systems, Sm2Sn2O2, Sm1.92Ca0.08Sn2O2-delta and Sm2Sn1.92Y0.08O7-delta was studied using impedance spectroscopy under wet and dry gas (O-2 and Ar) in the temperature range 150-1000 degrees C. Enhancements of the bulk conductivity of all samples at temperatures up to similar to 550 degrees C were observed for wet conditions consistent with significant levels of proton conduction. The presence of dissolved protons in the acceptor-doped materials, Sm1.92Ca0.58Sn2O7-delta and Sm2Sn1.92Y0.08O7-delta, is supported by infrared spectroscopy and thermogravimetric analysis. Proton conduction was confirmed by isotope effects under heavy water (O-2/D2O and Ar/D2O). The A-site substituted sample Sm1.92Ca0.08Sn2O7-delta yielded the highest levels of proton conduction and displayed mixed ionic and electronic conduction under dry oxidising conditions. Electron hole conduction dominates in dry oxygen for Sm2Sn1.92Y0.08O7-delta and Sm2Sn2O2. For the A-site doped sample bulk and grain boundary conduction could be separated. The specific grain boundary conduction was calculated using the brick layer model and was found to be two orders of magnitude lower compared to the bulk conductivity. The unexpected increase in conductivity seen for the undoped sample under wet gas is discussed in the context of structural disorder and possible filling of the un-occupied anion site in the pyrochlore structure by OH-groups. (C) 2010 Elsevier B.V. All rights reserved.