Molecular dynamics computer simulations were used to study diffusion of calcium ions in calcium alumino-silicate (CAS) intergranular films (IGFs) connecting two differently oriented alumina crystals. Diffusion coefficients and the activation energy for Ca diffusion in different parts of the IGF were calculated over four different IGF systems. The simulation results show that the diffusion coefficients adjacent to the interfaces are smaller and the activation energies are much higher than those in the interior of the IGF and in bulk glasses. Ca diffusion is greatly affected by the ordered structures at the basal and prism interfaces, while it is only slightly affected in the interior of the IGF in comparison to bulk glass behavior. The activation energies for Ca diffusion in the bulk-simulated glasses are similar to that found experimentally and slightly lower in the interior of the same composition IGF. The results suggest that Ca transport is mainly though the interior of the IGF and implies that diffusion would be significantly inhibited by sufficiently thin IGFs.