A novel dosimetry-based technique using EPR spectroscopy to determine X-ray beam quality is proposed. The radiation-sensitive material is made of a mixture of two polycrystalline substances with different X-ray absorption properties. The composite samples, consisting of polycrystalline lithium formate monohydrate and calcium formate, were prepared as pellets, X-irradiated, and analyzed with EPR spectroscopy. The ratio of the EPR signal amplitudes of the two constituents can serve as a measure of the X-ray beam quality given by the equivalent photon energy. The calculation of the signal amplitude ratio involves a reconstruction of the composite EPR spectrum. The logarithm of the signal amplitude ratio appears to be linearly correlated with the logarithm of the equivalent photon energy. The linear relationship can be used as a calibration for estimating the equivalent photon energy from the composite EPR spectrum. The composite material was used to investigate the changes in the equivalent photon energy in a Perspex phantom with increasing depth. When a 220 kV X-ray beam with an equivalent photon energy of about 100 keV was used, changes in the EPR signal amplitude ratio revealed a buildup of scattered photons with increasing depth in the phantom. This change could be related to the equivalent photon energy using the logarithmic calibration curve. It was found that the equivalent photon energy at the depth of 13 cm in the phantom was 25% lower than on its surface. The proposed method can be used for estimating equivalent photon energy in both standardized and non-standardized situations, the latter corresponding to beam setups where use of filters and ionization chambers is difficult or impossible. Also, the system can provide a means for measuring photon energy in X-irradiated phantoms.