This work aims to characterize mammographic x-ray beams incident and transmitted by breast phantoms (from 0 to 45 mm) composed from known proportion of glandular and adipose tissue-equivalent materials. This study was performed for mammographic x-ray beams generated by a mammography equipment using different target/filter combinations (Mo/Mo, Mo/Rh and W/Rh). It was studied the modification of spectra shape of the beams transmitted through different thicknesses of these materials. It was also evaluated the penetrability of these transmitted beams by its correlations to the HVL, which were experimentally estimated and derived from the x-ray spectra measured using a spectrometry system with a CdTe detector. The x-ray spectra transmitted by the phantom with higher density presented lower intensity than those transmitted by those with lower density, as expected. The differences between the HVL values derived from the spectra and those estimated using air kerma measurements are lesser than 6% for about 88% of the spectra measured in this work. The expected spectra variations with phantom thickness, revealed by the measured transmitted x-ray spectra, were also confirmed by HVL measurements and agree with the estimated attenuation curves. The motivation of the study was related to the robustness of the spectra as a descriptor of radiation beams and the possibility of using these transmitted spectra for dose assessment related to mammographic procedures. We can conclude that developed method is able to characterize mammographic x-ray beams making it possible the use of this kind of data for dose assessment in mammography.