An investigation of x-ray spectral reconstruction from transmission data by direct resolution of the matrix system A*F = T (using spectral algebra formalism) has been previously presented. The resolution has been done with simulated spectrum. In this paper, the method on a real case of a 12 MV photon beam was tested. A special study of the setup has been made to estimate and reduce the experimental errors that could alter the results. In order to convert F(E) (a fraction of the signal due to a photon of energy E) into photon fluence phi (E), the chamber energy response R(E) has been studied and an approximated analytical function for its representation was proposed. Spectra reconstructed from different transmission data using different attenuators, buildup caps, and ionization chambers have been compared to verify the uniqueness of the reconstructed spectra. To test the validity of the results, dosimetric values, such as Depth Dose Data have been calculated, from our spectrum, using a specific code developed by Kosunen et al. The results show a good agreement between the measured and calculated data.