We have prepared the (MgB2)(2-x)Co-x, superconducting system, where x = 0.0, 0.1, 0.3 and 0.5, by using conventional solid-state method. The XRD and SEM investigations showed that the addition of Co into the MgB2 system caused a multiphase formation with impurity phases MgB4 and MgCo2O. The Co concentration had considerable effect on the electrical properties. It was found that the T-c and T-0 values decreased when the Co concentration was increased in the system. The superconducting transition was obtained to be sharp for the x = 0.0 sample whereas the transition become broader with increasing the applied magnetic field and Co concentration. The carrier concentration, n, decreased drastically from 146 x 10(21) for x = 0.0 to 0.53 x 10(21) for x = 0.5 with increasing the addition of Co. The activation energy, U(H), and Fermi energy, E-F, of the samples prepared decreased with the applied magnetic field and the Co content. The thermoelectric power, S(T), value was obtained to be positive for all the samples, indicating that the hole-type conductivity is dominant in the materials prepared. When the Co concentration was increased the magnitude Of kappa was suppressed and sharp rising of kappa(T) at temperatures close to T-c which is often observed in the HTc systems was not obtained due to the scattering of electrons on impurities for the samples prepared in this study. (c) 2005 Elsevier B.V. All rights reserved.