We present the results of static lattice energy minimisation calculations investigating the defect structure of pyrochlore structured oxides of general formula A(2)B(2)O(7) (A = Gd,Y;B = Zr,Ti). Our calculations indicate that ionic motion will be effected predominantly by a vacancy hopping mechanism between 48f sites. We also show that cation disorder is necessary for the thermal generation of anion vacancies on these sites. We find low solution energies for certain aliovalent dopants confirming the possibility of generating substantial concentrations of extrinsic defects. Approximate calculations of the electronic band gaps, based on a small polaron model, give good agreement with measured values.