The high temperature structural behaviour of the uranium perovskites Ba₂CaUO₆ and BaSrCaUO₆ has been investigated using a combination of synchrotron X-ray and neutron powder diffraction. Ba₂CaUO₆ undergoes a complex sequence of structures associated with the progressive loss of cooperative octahedral tilting: P2₁/n → I2/m → I2/m → I4/m → Fmm. The observation of the intermediate tetragonal structure, I4/m, in this, contrasts with the previously reported rhombohedral R intermediate formed by the Ba₂SrUO₆ oxide. The importance of π-bonding in determining the structural sequence is discussed.