The quest for novel solid electrolytes and mixed ionic electronic conductors is leading us to attempt aliovalent cation substitution of new classes of oxides, but in a number of cases such doping gives no or unexpected effects. A general feature of many such systems is that they are ternary or higher oxides in which two cations of different valence are site disordered, facilitated by similar size. Examples comprise TiNb2O7, TiP2O7, ZrP2O7 and Ba3La(PO4)(3). Doping intended to create charge compensating mobile point defects may in these cases result only in shifting the ratio of the two disordered cations, as this may have a much lower energy cost. This is accompanied by the precipitation of a phase or domains rich in the expelled cation. Entropy and Gibbs energy changes by doping are calculated and compared for cation disordered and non-disordered systems for illustration. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.