The distribution of alkaline earth ions (R = Ca, Sr, Ra) in beta"-alumina has been studied using static lattice computer simulations. It is found that the conduction plane becomes narrowed and the c-axis of the structure shortened with the substitution of Na by Ca, due to a significant displacement of the bridging oxygen, 0(5), towards the Ca ion. Such a disordering of 0(5) is much less significant in Sr or Ba beta"-alumina. Whilst the seven-coordinated Beevers-Ross (BR) site is the preferred position for Ba, Ca shows a tendency to sit close to the eight-coordinated mid-oxygen (mO) site, which is in agreement with previous X-ray diffraction studies. Sr, on the other hand, shows a behavior intermediate between Ca and BE, i.e. it shows a preference for the mO site but can also stay in the BR site, depending on the configuration of the surrounding Na vacancies. Symmetric defect configurations with respect to the BR site result in the occupation of the BR site by all the R ions, whereas asymmetric configurations lead to a mO site occupation by Ca or Sr. Short range ordering of alkaline earths in the conduction plane was also observed from the trend that two R ions prefer to stay apart. The calculated results show that the site preference of R in the beta＇＇-alumina structure largely depends on alkaline earth ionic radius, which is in good agreement with the physical property measurements.