The ultraviolet (UV) photoluminescence (PL) properties of Tm-doped Alx Ga1-x N (0.39x1) alloys grown by solid-source molecular beam epitaxy were probed using above-bandgap excitation from a laser source at 197 nm. The PL spectra show dominant UV emissions at 298 and 358 nm only for samples with x=1 and 0.81. Temperature dependence of the PL intensities of these emission lines reveals exciton binding energies of 150 and 57 meV, respectively. The quenching of these UV emissions appears related to the thermal activation of the excitons bound to rare-earth structured isovalent (RESI) charge traps, which transfer excitonic energy to Tm3+ ions resulting in the UV emissions. A model of the RESI trap levels in AlGaN alloys is presented.