The (MgB2)(2-x)CUx (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E-F, and the corresponding velocity, V-F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB2. Two different impurity phases, MgCu2 and CuB24, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T-c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x = 0.0 samples, the transition is found to be sharp, Delta T similar to 1K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x = 0.0 sample the n was calculated to be 12 X 10(21) cm(-3), but for the x = 0.5 sample it decreased to 1.3 X 10(21) cm(-3). We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, EF, calculations the decrease of the carrier concentration by the additions of Cu into MgB2 gives a decrease in EF and this could be attributed to a shift of the Fermi level towards the top of the sigma-hole band. (c) 2005 Elsevier Ltd. All rights reserved.