The exploration of the phase diagram of the ferrite YBaFe4O7+δ versus the oxygen content δ and temperature shows the complex crystal chemistry of this system. Besides the cubic form (F4¯3m), which is observed up to 600 °C and for 0<δ≤0.65, a stoichiometric tetragonal form (δ=0) is isolated below 300 °C that is stable only in the absence of oxidizing atmosphere. The resolution of the structure of this new YBaFe4O7.0 form, from combined neutron and synchrotron data, in the space group I4¯, shows significant displacements of the atoms with respect to the cubic form, especially concerning the oxygen atoms surrounding the barium cations. The decrease of several Ba–O distances around the underbonded barium cations is explained by the existence of hybridized Ba(2−δ)+–O2−−Fe(2+δ)+ bonds, in agreement with Mössbauer spectroscopy. The role of coulombic repulsions in the [Fe4O] and [Fe4] tetrahedra of the [Fe4]∞ sublattice on the structural transition is also discussed.