The crystal structure and electrical properties of stoichiometric perovskite proton conductors SrCe1-xYxO3-delta (where x = 0.025, 0.05, 0.075, 0.1, 0.15, and 0.2 and delta = x/2) and substoichiometric Sr0.995Ce0.95Y0.05O3-delta have been investigated. The conductivities of the samples were measured as a function of the partial pressure of oxygen at 600 and 800 degreesC, and at two water vapor pressures (P-H2O = 0.01 and 0.001 arm). A P-O2 range of 1 atm (pure O-2) to approximately 1 x 10(-25) atm (N-2/H-2 mix) allowed for the separation of n-(electron), p-(hole), and i-(ionic) type conductivities. In the case of stoichiometric perovskite proton conductors, the unit cell volume (UCV) and calculated density decrease with increasing yttrium content. The ionic and p-type components of the conductivity show threshold effect with Y-doping, which may be related to the double substitution of Y on both A- and B-sites. A maximum ionic conductivity of 5 mS/cm is found at 10% Y, whereas p-type conductivity increases with increasing yttrium concentration. A conductivity component appearing at low oxygen partial pressures decreases with yttrium doping. The substoichiometric material showed a drop in unit cell volume of approximately 0.34 Angstrom(3) compared to its stoichiometric partner. The conductivity components of substoichiometric material are higher than the conductivities of corresponding stoichiometric material, being approximately 7 mS/cm for both P-H2O levels. (C) 2004 Elsevier B.V. All rights reserved.