The powder and bulk MgB2 superconductors sintered in different Ar gas pressures were investigated using X-ray diffraction patterns, mass density and mass attenuation coefficient measurements. During the sintering process, 2, 4, 6, 8 and 10 bar argon pressures were used to minimize the evaporation of Mg from the compound. Mass attenuation coefficients (mu/rho) of powder and bulk samples were determined by performing transmission experiment in narrow beam geometry using a collimated X-ray beam at 14.1, 15.8, 17.4, 19.6, 22.1, 24.2, 24.9, 26.3, 27.2, and 29.7 keV energies. It was found that the mass attenuation coefficients decreased with increasing photon energies. The coefficients were compared with tabulations based upon the results of XCOM. A good agreement has been observed between experimental and theoretical tabulations. In addition, the density of bulk sintered samples decreased with increasing Ar pressure because of the removal of Mg atoms from the compound. (c) 2006 Elsevier Ltd. All rights reserved.