The ionic and electronic conductivities of samaria doped ceria electrolytes, Ce0.85Sm0.15O1.925-delta, with nanometric grain size have been evaluated. Nanostructured bulk specimens were obtained using a combination of high specific-surface-area starting materials and suitable sintering profiles under conventional, pressureless conditions. Bulk specimens with relatively high density (>= 92% of theoretical density) and low medium grain size (as small as 33 nm) were achieved. Electrical A. C. impedance spectra were recorded over wide temperature (150 to 650 degrees C) and oxygen partial pressure ranges (0.21 to 10(-31) atm). Under all measurement conditions the total conductivity decreased monotonically with decreasing grain size. In both the electrolytic and mixed conducting regimes this behavior is attributed to the high number density of high resistance grain boundaries. The results suggest a possible variation in effective grain boundary width with grain size, as well as a possible variation in specific grain boundary resistance with decreasing oxygen partial pressure. No evidence appears for either enhanced reducibility or enhanced electronic conductivity upon nanostructuring. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.056205jes]