BaZr0.9In0.05M0.05O3-delta (M = Ga3+ or Yb3+) has been prepared by solid-state synthesis route. Rietveld analysis of neutron powder diffraction data on as-prepared samples showed that both samples crystallise in the cubic space group Pm (3) over barm. Scanning electron microscopy analysis showed that the grains in the BaZr0.9In0.05Ga0.05O3-delta sample were larger than the BaZr0.9In0.05Yb0.05O3-delta sample. Dynamic thermogravimetric analysis indicates that the proton concentration was higher in pre-hydrated BaZr0.9In0.05Ga0.05O3-delta sample (40% of theoretical) than that of BaZr0.9In0.05Yb0.05O3-delta sample (28% of theoretical). Proton conductivity was studied on pre-hydrated samples (under both dry and wet Ar atmospheres). The bulk proton conductivities of the heating cycle of pre-hydrated BaZr0.9In0.05M0.05O3-delta (M = Ga3+ or Yb3+) samples were 2.1 x 10(-5) S cm(-1) and 1.9 x 10(-4) S cm(-1) at 350 degrees C, respectively. The effect of co-doping on proton conductivity was investigated and the results are compared with single doped systems e.g. BaZr0.9M0.1O3-delta (M = Ga3+, In3+ or Yb3+) samples. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.