The crystal unit cell of mayenite, 12CaO.7Al2O3, consists of a cubic lattice of [Ca24Al28O64]4+ with the remaining 2*O2- contained in cages, which are linked by channels within the structure. This material has been termed an 'inverse' zeolite in so far as it contains charge compensating anions within the pores of the positively-charged framework; indeed hydroxide and halide ions and even electrons can be incorporated to provide charge neutrality. The linked nature of the voids in the structure allow oxygen-ion conduction and this conductivity is enhanced at high temperature. This proposal aims (i) to identify the location of these extra-framework O2- anions; (ii) to determine their effect on the neighbouring framework environment and (iii) to see how structural changes with increased temperature can be related to the increases in ionic conductivity.