The potential of the perovskite system Nd1s,,Sr,,Co0 3_6 (x = 1 and 2/3) as cathode material for solid oxide fusel cells (SOFCs) has been investigated ina detailed structural, electrical, and electrochemica1 characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of inte rgroBin, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the 0.4 temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovslr.ite with a homogeneous x icrostriicture and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation.temperature. Both materials_ are p -type electronic conductors with high total conductivities of 690 and 1675 S"cm at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more ocygen on heating than x = 1/3, associated with a greater loss of p -type charger carriers. However, composite cathodes prepared with Nd/ Sr2s3Co03_6 and Ce0,8Gd0202_6 present lower ASR values (0.10 t2.cm2 at 973 K in air) than compositesprepared with Nd21/33Srii3C(>03 3 and Ce08Gdo.202_6 (0.34 52.cm2). The high activity for the oxygen electrochemical reactionat intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very devices.