A simple model is used for predicting the way cations are distributed among the known crystallographic sites I, I', H, and III, in NaY and NaX faujasite. zeolites. This model is based on the well-known assumption that the cation distribution is dominated by the repulsive cation-cation Coulombic interactions. In the case of NaY the predicted distributions are in good agreement with the available experimental data. Monte Carlo simulations have been performed, using a simplified framework model together with the recently proposed cation-framework model of Jaramillo and Auerbach. These simulations confirm the basic assumption of the cation-filling model. They also reveal a striking sensitivity of the configuration energy to a weak deviation from the most uniform local cation spatial distribution. The existence of the extra site III' experimentally reported in NaX is also observed in these simulations.