More careful analysis and changes in synthetic recipes can potentially lead to the successful synthesis of the predicted phases in the future. For the double perovskites KBaGdWO 6 and KBaPrWO 6 , the fits are so poor as to be meaningless in our view, with clear unfitted Bragg peaks in both cases. Both these structures have order on both the A and B sites (so called double-double-perovskites). A site order in perovskites is much rarer than B site ordering, and is almost always accompanied by oxygen deficiency, 32 a classical example is the cuprate superconductor YBa 2 Cu 3 O 7 . In the absence of oxygen deficiency, A site order can rarely occur but has to our knowledge not been seen in the symmetry predicted here -typically A sites form ordered layers driven by large charge differences, rather than fcc patterns. Given the novelty of what is being proposed here, clear evidence must be given that the predicted compounds have been formed, with special attention paid to small diffraction features that characterise the ordering. This evidence is not supplied by the poor fits to the PXRD data.