The electrical conductivity of sintered samples of (CeO2)(1-x)(SmO1.5)(x) (0.10 less than or equal to x less than or equal to 0.40) in air was investigated systematically as a function of temperature between 240 and 1000 degreesC by using two-probe ac impedance technique. The relative contribution of the grain interior (lattice) and grain boundaries to the overall resistance, is discussed in detail in relation with the grain boundary effect. The maximum lattice conductivity observed (at 600 degreesC) was 2 S/m for Ce0.9Sm0.1O1.95. With an increase in samarium 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 degreesC) = 1 S/cm) in the overall conductivity at x = 0.2. A linear relationship between the association enthalpy of the defect cluster, [Sm-Ce' - V-O(. .)], and x(1/3) was proposed.