The crystals of yttrium–aluminum garnet with an admixture of iron and cerium have been studied by the method of wideband EPR spectroscopy. Resonance absorption of microwave radiation in the frequency range 220–300 GHz was detected. Based on the experimental data set, it is concluded that the signals belong to the Mo^3+ ion, which is included in the crystals as an uncontrolled impurity. A straightforward method was used to measure the splitting in a zero field between the Kramers doublets |±1/2〉 and |±3/2〉 (261.6 ± 0.5 GHz). The calculated angular and frequency-field dependences of the EPR spectra are in good agreement with the experiment.