In this paper, we discuss different possible cation diffusion mechanisms in YSZ and LSGM. Monte Carlo simulations are also reported of tracer diffusivities in LSGM for a postulated cluster mechanism. These simulations extend earlier simulations on un-doped material. The limits of the ratio of the diffusivities are consistent with experimental tracer diffusion findings over a wide range of cation-vacancy exchange frequencies. We also develop relationships between the phenomenological coefficients and use these relationships to predict possible demixing and interdiffusion experimental outcomes.