The basic expressions governing the ionic conductivity of fast ion conductors are examined. Acceptor doped oxides displaying fast ion conduction are taken as an example. For selected materials, the role of dopant-vacancy interactions in influencing the concentration of mobile vacancies is assessed. Examination of experimental data and the results from atomistic lattice simulations lead to the following conclusions. For acceptor dopants where the effective charge of the substitutional ion is - 1, a minimum is seen in the concentration dependence of the activation energy for oxygen ion conduction. This minimum is a characteristic feature and can be used as an indicator of dopant-vacancy interactions. The activation energy for conduction is also dependent upon the size of the dopant, through a size dependence of the association enthalpy of the dopant-vacancy pairs. The activation energy is minimised when the dopant is close to the size of the host cation. A particular example of materials where this is optimised is the Sr'(La) substitution found in many of the mixed conducting perovskites, which may go part way in explaining the very high diffusivities of oxygen found for these materials.