This paper describes the water adsorption behavior and electrical conductivity of Sm-doped ceria nanoparticles under humidified atmosphere. Single nano-sized 6 mol% Sm-doped ceria was prepared by either homogeneous precipitation using hexamethylenetetramine (HMTA) or ultraviolet (UV) laser irradiation. For the ceria nanoparticles prepared by HMTA, weakly-bundled agglomerates with pores were formed; as a result, approximately 3 mass% of water adsorption and capillary condensation were observed at room temperature. On the other hand, those prepared by UV laser irradiation formed tightly-bundled agglomerates without such pores; only 0.12 mass% of water was adsorbed under the same condition. For electrical conductivity measurements, dense samples having a relative density of 85 to 93% were prepared at 300 degrees C under 6 GPa by using a high-pressure apparatus. The proton conductivity of dense nanocrystalline ceria was on the order of 10(-7) to 10(-8) S/cm at around 50 degrees C under humidified atmosphere. As temperature increased, the proton conductivity decreased. (C) 2009 Elsevier ay. All rights reserved.