Dense nanocrystalline Ce0.9Gd0.1O2-delta (nCGO) with a median grain size of similar to 120 nm was prepared by spark-plasma sintering of nanoscaled powders. The electrical behaviour of nCGO was analysed by impedance spectroscopy and compared with the micro-grained material of the same composition in humidified (H2O Or D2O) and dry O-2 in the temperature range 25-600 degrees C. The large volume of grain boundaries in the nanometric material is highly blocking to oxide-ions, the majority charge carriers above 100 degrees C, such that the impedance response in this range is dominated by the grain-boundary contribution. The much smaller grain-boundary resistance of micrometric CGO is attributable to the larger grain size rather than a different grain-boundary thickness or conductivity. Proton transport dominates the electrical conductivity of nCGO in wet atmospheres below 100 degrees C, as demonstrated by the presence of a conductive H+/D+ isotope effect. The absence of a measurable electromotive force in a water-based concentration cell with nCGO as separating membrane and a massively higher resistivity of nCGO samples with a blocked lateral surface both strongly indicate that the proton transport is attributable to surface processes associated with chemisorbed and physisorbed water layers rather than grain-bulk or grain-boundary phenomena. The magnitude of the room-temperature surface proton conductivity is 4-5 times greater for nanostructured Ce0.9Gd0.1O2-delta than the micrometric analogue. (C) 2009 Elsevier B.V. All rights reserved.