Oxides with proton conductivity have a great potential for applications in environmental energy technology. Despite the BaCe0.4Zr0.4Y0.2O3-δ (BCZY) perovskites are well-known proton conductors, it is a challenge to determine the optimal operat-ing temperature range where the energy applications benefit most from this unique property. The protonic transport proper-ties strongly depend on crystal structure and local distortions in the participating cation coordination sphere, according to related temperatures and gas feed. The transport and crystallographic properties of BCZY were simultaneously studied by Impedance Spectroscopy (IS) and Synchrotron X-Ray Diffraction (S-XRD). A strong correlation between conductivity and lattice parameter, corresponding in principle to a cubic symmetry, was observed, mainly between 400 and 700 °C. The pro-tonic conductivity range was analyzed by the H/D isotopic effect on the impedance spectra, which helped to identify pro-tonic conduction as the governing transport mechanism below 600 °C, while the transport via oxygen vacancies dominates above this temperature. In order to assess the real crystallographic structure, the simultaneous refinement of laboratory XRD and Neutron Diffraction (ND) patterns was performed. According to this, BCZY changes from rhombohedral sym-metry below 400 °C to cubic at 600 °C in a second order phase transition. Complementary Quasielastic Neutron Scattering (QENS) enables to determine a protonic jump length of 3.1 Å, which matches the O-O distances in the octahedral oxygen coordination sphere around the cations. These results support the protonic self-diffusion through proton hopping between intra-octahedral O sites as the main transport mechanism up to 600 °C.