Sm-doped ceria is a candidate electrolyte for SOFCs. The electrical conductivity of sintered samples of (CeO2)1-x(SmO1.5)x (x = 0.10-0.40) in air was investigated as a function of temperature between 240 and 1000 C. The relative contribution of the grain interior (lattice) and grain boundaries to the overall resistance, is discussed in relation to the grain boundary effect. The maximum lattice conductivity observed (at 600 C) was 2 S/m for Ce0.9Sm0.1O1.95. With an increase in Sm concentration, the grain interior conductivity decreases gradually, but the grain boundary conductivity increases dramatically. The two opposite effects lead to a maximum (sigma(t,600 C) = 1 S/cm) in the overall conductivity at x = 0.2. A linear relationship between the association enthalpy of the defect cluster and x exp(1/3) was proposed. 24 refs.