The effect of co-doping LaCoO₃ with Ni and Nb has been investigated in the two solid solution series (1 − x)LaCoO₃–xLaNi_2/3Nb_1/3O₃ and (1 − y)LaCo_2/3Ni_1/3O₃–yLaNi_2/3Nb_1/3O₃. The materials were shown to be stoichiometric with respect to oxygen. The compositional effect on the evolution of the crystal structure, thermal expansion and electrical conductivity is reported. The chemical stability of LaCoO₃ in a reducing atmosphere was improved and the thermal expansion was suppressed at higher levels of Nb-substitution. The materials with mixed oxidation states on the B-site possessed superior electrical transport properties compared to solid solutions with cations in mainly isovalent oxidation states. The evolution of the electronic conductivity, thermal expansion and crystal structural is discussed with emphasis on the oxidation states of Ni and Co, and the spin transitions of Co.