Nanostructured samaria- and gadolinia-doped ceria (SDC and GDC) powders were synthesized at low temperature (400 degrees C) using diamine-assisted direct coprecipitation method. Fast-firing (f.f.) processes, where sintering temperatures are reached in a short time to promote lattice diffusion, were compared with conventional sintering, for the formation of dense microstructures from the nanostructured powders. Highly dense SDC and GDC samples (96%) with reduced grain size (150 nm) were obtained by f.f. even at 1300 degrees-1400 degrees C and, unexpectedly, high electrical conductivity and low blocking effect at grain boundary was obtained. Conventionally sintered samples showed that the grain boundary resistivity decreased with increasing the grain size, in agreement with the increase in geometrical bulk volume/grain boundary area ratio. Conversely, f.f. samples showed grain boundary resistivity smaller for small grain size. The above effect was observed only for high dopant (> 10% molar) contents. The combined effect of powder grain size, dopant content, and sintering temperature-time profile, can be exploited to tune ceria microstructures for specific ionic device applications.