The proton conductivity in tin-based pyrochlores, Ln(1.96)Ca(0.04)Sn(2)O(7-delta) (Ln = La, Sm, Yb), has been investigated. Samples were prepared by conventional solid state sintering methods. Fractions of the powders were vacuum dried, protonated and deuterated so that infrared spectroscopy and thermogravimetric analysis could be carried out to study proton incorporation. Electrochemical impedance spectroscopy was used to characterise the conductivities of the sample. The bulk conductivity was higher in wet gas compared to the dry runs at temperatures below approximately 500 degrees C for all samples. The level of proton conductivity was found to depend critically on the lanthanide size, and was highest for the largest La and mid-sized Sm ions, sigma(H+) approximate to 4 x 10(-7) Scm(-1) at 300 degrees C, before falling sharply for the Yb-ion, sigma(H+) approximate to 7 x 10(-9) Scm(-1). Evidence from TGA and IR analyses indicates that the tendency for proton absorption was strongly influenced by the lanthanide size, being greatest for La1.96Ca0.04Sn2O7-delta. The results suggest that both proton concentration and proton mobility vary depending on the size of the A-site ion. (C) 2010 Elsevier B.V. All rights reserved.