We determined the energies of the Mg L2,3, Sn N4,5, and In N4,5 attenuation edges by measuring the transmission of high-resolution synchrotron radiation through thin filters. The Al L2,3 and Si L3 edges observed in the first and higher diffraction orders from the monochromator were used as energy fiducials. For each attenuation edge, the onset of attenuation with increasing energy and the inflection point of the attenuation curve were measured. The measured energy values were compared with previously determined attenuation edge energies and with electron binding energies. The measured energies of the inflection points are Mg L2 (49.89 +/- 0.02 eV), Mg L3 (49.58 +/- 0.02 eV), Sn N4 (25.00 +/- 0.02 eV), Sn N5 (23.97 +/- 0.02 eV), In N4 (17.66 +/- 0.02 eV), and In N5 (16.70 +/- 0.02 eV).