Double B perovskite-type series in the system La0.6Ca0.4Mn1-xO3-delta with x=0-0.6 was synthesized for the first tin-le in air by solid state reactions at 1200 degrees C from simple oxides and CaCO3. Single phase compositions with small impurities of NiO-MnO solid solutions were formed. Replacement of Mn cations with Ni cations is accompanied by strong increase in electrical conductivity of the prepared ceramics. Lower conductivities of the Ni containing compositions compared to La0.6Ca0.4.MnO3-delta, are explained by stronger polarization of the -Mn(alpha+gamma)+- O alpha--Ni(alpha-gamma)+-O alpha-- fragments of the -O alpha--Mn alpha+ -O alpha--Mn(alpha+gamma)+O alpha--Ni(alpha-gamma)+-O alpha- chains in comparison with the O alpha--Mn alpha+ -O alpha--Mn alpha+-O alpha- chains without Ni-cations, because of different electronegativity values of Nii and Mn. The compositions with x=0-0.1 and x=0.6 are p-type and those with x=0.2-0.5 are n-type semiconductors. Type and level of the electrical conductivity of the La0.6Ca0.4Mn1-x series are the functions of the [Mn (4+)]/[Mn3+] and [Ni3+[Ni2+] ratios. Mn4+ Ni4+ and Ni2+-cations are possible point defects which determine the p- and n-type conductivity of compositions, respectively. (c) 2007 Elsevier B.V. All rights reserved.