This work is aimed to a comprehensive study of the role of the microstructure and of the local chemistry at the grain-boundaries on the ionic grain-boundary conductivity in yttria (9mol%)-stabilized zirconia (YSZ) and in YSZ-alumina composites. Electrical conductivity measurements show that the grain-boundary conductivity (σ_gb) increases with the cooling rate at the end of sintering, when the grain size and the amount of Si in the ceramic decreases. Alumina additions lead to a decrease of σ_gb in the samples containing 1.0 wt% SiO₂, while σ_gb passes through a maximum for heavier silicon contents. These results, coupled to TEM X-ray microanalysis, suggest that alumina nanoparticles interact strongly with SiO₂ at the grain boundaries giving rise to a competitive effect: whereas the alumina nanoparticles increase the insulating character, they also reduce the vitreous phase at the grain boundaries, increasing the conductivity. Moreover, XPS analyses show that Si and Y segregate near the interfaces, on a depth smaller than 600nm. These analyses suggest a kinetic demixing process and they allow to explain the positive effect of a faster cooling rate at the end of sintering, responsible for the smaller amount of Si rejected at the grain-boundaries.