BaSnO3 is a relatively new family of proton-conducting materials, which are attractive for high-temperature applications, including protonic ceramic fuel cells and protonic ceramic electrolysis cells. In this work, we synthesized the BaSn1–xYxO3–δ (0 ≤ x ≤ 0.4) phases and provided their in-depth characterization utilizing high-temperature X-ray diffraction and dilatometry techniques to reveal the fundamental regularities in the variations of chemical and thermal strains depending on composition. It is found that chemical expansion/contraction effects become to be more important with increasing the Y-content. In particular, the weakly doped stannates exhibit predominantly thermal expansion, while the heavily doped stannates (especially, x = 0.4) display a notable chemical contribution. The mentioned effects are discussed in terms of the BaSn1–xYxO3–δ defect structure and its ability towards hydration and dehydration. This work therefore provides valuable data for the real application of the studied materials (in both powder and ceramic forms) as well as other pronounced proton-conducting electrolytes. © 2023 Elsevier Ltd and Techna Group S.r.l.