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Gallium oxide (Ga2O3), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga2O3 crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga2O3. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies.