Comparative studies were performed on variations in the ABO₃ perovskite structure, chemical stability in a CO₂-H₂ gas atmosphere, and electrical conductivity measurements in air, hydrogen, and humidity-involving gas atmospheres of monophase orthorhombic Ba_1−xSr_xCe_0.9Y_0.1O_3−δ samples, where 0 < x < 0.1. The substitution of strontium with barium resulting in Ba_1−xSr_xCe_0.9Y_0.1O_3−δ led to an increase in the specific free volume and global instability index when compared to BaCe_0.9Y_0.1O_3−δ. Reductions in the tolerance factor and cell volume were found with increases in the value of x in Ba_1−xSr_xCe_0.9Y_0.1O_3−δ. Based on the thermogravimetric studies performed for Ba_1−xSr_xCe_0.9Y_0.1O_3−δ, where 0 < x < 0.1, it was found that modified samples of this type exhibited superior chemical resistance in a CO₂ gas atmosphere when compared to BaCe_0.9Y_0.1O_3−δ. The application of broadband impedance spectroscopy enabled the determination of the bulk and grain boundary conductivity of Ba_1−xSr_xCe_0.9Y_0.1O_3−δ samples within the temperature range 25–730 °C. It was found that Ba_0.98Sr_0.02Ce_0.9Y_0.1O_3−δ exhibited a slightly higher grain interior and grain boundary conductivity when compared to BaCe_0.9Y_0.1O_3−δ. The Ba_0.95Sr_0.05Ce_0.9Y_0.1O_3−δ sample also exhibited improved electrical conductivity in hydrogen gas atmospheres or atmospheres involving humidity. The greater chemical resistance of Ba_1−xSr_xCe_0.9Y_0.1O_3−δ, where x = 0.02 or 0.05, in a CO₂ gas atmosphere is desirable for application in proton ceramic fuel cells supplied by rich hydrogen processing gases.