Understanding the strain from chemical expansion of proton-conducting ceramics is crucial to prevent their failure under operating conditions. BZY10 (BaZr0.9Y0.1O3-) and BZCY72 (BaZr0.7Ce0.2Y0.1O3-) were studied using high-temperature X-ray diffraction (HT-XRD) in moist and dry reducing atmosphere in order to prevent the competition with the oxidation reaction. Both powder and dense specimens were investigated and similar lattice parameters were obtained, demonstrating that the data were recorded on samples that were in equilibrium with the surrounding environment. Two sets of experiments were performed. In the first one, the sample was hydrated in situ in the XRD chamber at high temperature and diffraction data were collected during cooling. For the second set, the samples were pre-hydrated ex situ in an autoclave and the XRD patterns were recorded during heating under dry conditions. As expected, lattice parameters were larger for the hydrated samples, due to hydration chemical expansion. The chemical expansion was also found to be larger with the presence of Ce. Finally, larger concentrations of protonic defects were present in the lattice of the ex situ pre-hydrated samples compared with the in situ hydrated samples.