The highly accurate core level binding energies measured by Fermi‐level Referenced ESCA (FRESCA) have been compared with those of Auger Electron APS (AEAPS) and XPS. The FRESCA and AEAPS measurements directly measured energies with respect to the Fermi‐level and are not subject to the instrumental uncertainties characteristic of XPS. Absolute total energies of photoelectrons relative to the Fermi level are measured to an accuracy of 0.12 eV and the energy of photoelectron peaks is measured to 0.17 eV by FRESCA. The accuracy of the binding energy measurements by AEAPS is about 0.35 eV. These accuracies permit the first meaningful opportunity to compare appearance potential and photoelectron binding energies as a test of possible fundamental differences in the two types of measurement. For the 3dtransition metal series 2p 3/2 binding energies no differences within the uncertainties are found for Ti, V, and Cr, while a significant difference exists for Mn, Fe, Co, and Ni.