An alternative approach is used to measure normalized mass attenuation coefficients (mu/rho) of materials with unknown thickness and density. The adopted procedure is based on the use of simultaneous emission of K-alpha and K-beta X-ray lines as well as gamma peaks from radioactive sources in transmission geometry. Cd-109 and Co-60 radioactive sources were used for the purpose of the investigation. It has been observed that using the simultaneous X- and/or gamma rays of different energy allows accurate determination of relative mass attenuation coefficients by eliminating the dependence of mu/rho on thickness and density of the material. (C) 2014 Elsevier Ltd. All rights reserved.