Transmission electron microscopy, XPS analysis, electrical conductivity and diffusion measurements were used to characterize the transport properties and grain boundary segregation phenomena in 9 mol% yttria-stabilized zirconia (YSZ). The highest grain boundary electrical conductivity (σgb) and oxygen diffusion coefficient (DO) values are shown by samples with a cleaner microstructure, sintered at 1600 °C and rapidly cooled at the end of sintering. XPS measurements show that an yttrium and silicon kinetic demixing process takes place during cooling. The amount of silicon rejected in the grain boundaries decreases when the cooling rate at the end of sintering increases, in agreement with the highest grain boundary conductivity values of the quenched samples. A formal treatment has allowed us to show that these results can be explained by the two competing effects occurring during cooling: the cation redistribution kinetics and the cooling rate.