A series of solid solutions in the La0.8Sr0.2MnO3 +/-delta (LSM)-xLa(0.95)Ni(0.6)Fe(0.4)O(3-delta) (LNF) system were synthesised by solid-state reaction to investigate the effect of Ni/Fe substitution for Mn on their DC electrical conductivity, structure and thermochemical stability. A non-linear change in structure, conductivity, magnetic properties and thermochemical stability of solid solutions was observed with the increase in the Ni/Fe content. A new phase (100 - x) LSM center dot xLNF with orthorhombic symmetry was found in the concentration range 35 <= x < 70 mol%, and exhibits the lowest value of the conductivity and the highest activation energy in the single-phase concentration range 43 <= x <= 53. This is, most probably, caused by a low concentration of Mn3+-O-Mn4+ units available for small polaron motion through the B sublattice. The compositions containing more than 50 mol% Ni in the B sublattice shows metallic-like conductivity behaviour. The temperature of the semiconducting metallic transition decreases with increase in the Ni concentration. All compositions had good thermochemical stability in air, in contrast to that under reducing conditions. The higher the Ni/Fe content in the sample, the faster the reduction of the solid solutions occurred in non-humidified Ar-H-2 (95 : 5) atmosphere. The overall oxygen stoichiometry of the solid solutions is discussed.