Rare earth-doped BaZrO3 is a very attractive material in electrochemical applications due to its proton conductive property. In this work, powder X-ray diffraction patterns of BaZr0.8M0.2O3−δ (M = Sc, Eu, Sm, Dy) were collected using synchrotron radiation, and also using characteristic X-ray of CuKα in dry and wet atmospheres at high temperature. Then, a combined interpretation of the diffraction patterns was established by using Rietveld refinement. The results revealed that an obvious lattice expansion was observed for BaZr0.8M0.2O3−δ (M = Sc, Eu, Sm, Dy) in wet O2 compared with the case in dry condition, indicating a chemical expansion effect on lattice volume by incorporating water into lattice. Eu, Sm, and Dy cations occupied both A- and B-sites of BaZrO3 crystalline lattice, whereas Sc cations were determined to occupy B-site only. These results indicate clearly an increasing tendency toward A-site occupation for the rare earth cations in BaZrO3 with an increasing radius.