The last couple of decades have seen a rapidly increasing interest in new environment friendly energy sources and one key aspect of this research involves characterisation of new solid ceramics with high ionic conductivity, i.e. solid electrolytes. We propose to investigate a series of anion-deficient oxide fluorites for determination of defect structure and vacancy clustering by means of ¿total scattering¿ (Bragg peaks plus diffuse scattering). The clustering of vacancies is a serious obstacle for high ionic conductivity and increased knowledge about this is crucial for the further development of solid electrolytes.