The co-substituted Ca(1-x)Ln(x)Mn(1-x)Nb(x)O(3) (Ln =Yb, Lu; 0.02 <= x <= 0.08) are synthesized by solid-state reaction and the electronic transport properties are investigated. Rietveld refinement confirms the formation of single phase orthorhombic structure with gradual increase of cell parameters with doping level. The electronic transport properties such as Seebeck coefficient and electrical resistivity decrease with increasing the dopant concentration for both the co-substituted compositions. All the compositions of Ca(1-x)Ln(x)Mn(1-x)Nb(x)O(3) show nonmetal-like temperature dependence of resistivity; whereas metal-like temperature dependence of thermopower. This inconsistency is explained by the formation of oxygen vacancy associated defect centres that originates from partial reduction of Mn4+ to Mn3+ due to co-substitution. The defect centres act as extrinsic carriers and cause additional contribution to the entropy of the system, leading to increase of Seebeck coefficient as a function of temperature. The transport mechanism of charge carriers is explained in the framework of Mott's small polaron hopping mechanism. (C) 2015 Elsevier Ltd. All rights reserved.