The microstructure, lattice parameters, mechanical properties, and electrical conductivity mechanisms for Co, Mn co-doped (La0.8Ca0.2)CrO3-delta have been systematically investigated. In this study, the concept of defect chemistry is used to explain the relationship between the compensation mechanisms and the electrical conductivity. In air, the hole concentration at high oxygen activity (p = 0.2-u) is larger than that at low oxygen activity (p = 0.2-u-2 delta(1)). Therefore, the electrical conductivity decreased with increasing Mn-doping level, and the compensation mechanism is significantly dominated by electrical compensation. In 5% H-2-95% Ar, the transformation temperature occurred in Mn-doped (La0.8Ca0.2)(Cr0.9Co0.1)O3-delta specimens, which indicates that the prevailing compensation mechanism is changed. Above the transformation temperature, the prevailing compensation mechanism changes from electrical to ionic compensation. The effect of atmospheres on fracture toughness and microhardness for specimens is also investigated in this study. It is found that when the specimens were exposed to 5% H2-95% Ar at 1000 degrees C, the cracks appeared in Mn-doped (La0.8Ca0.2)(Cr0.9Co0.1)O3-delta specimens. Presumably, the ionic radii were changed and generated stress that increased brittleness and leaded to cracking. With the increase in the Mn-doping level, the degree of cracking was increased. This indicates these materials could not be used as interconnects at high temperature in the reducing atmosphere. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610215] All rights reserved.