High resolution neutron diffraction patterns of BaCexZr1−xO3 (x = 0, 0.1, 0.4, 0.8) were obtained at various temperatures. The phase diagram that was deduced from Raman measurements has been confirmed. Structural transitions occur in a fixed order Pnma-Imma- R c-Pm3m as a function of temperature or composition. For BaCeO3, the large volume change that has been previously claimed at the Imma-R c transition has been revisited and found inconsistent. For a given composition, the cell volume increases when temperature increases, but the MO6 (M=Ce-Zr) octahedron volume decreases. It is shown that the ratio of the cell volume to the octahedron volume is a good indicator of the phase transitions. Transitions occur at fixed values (5.77, 5.80 and 6 for the Pnma-Imma, Imma-R c and R c-Pm3m transitions respectively) independently of composition.