Core-hole-clock spectroscopy is increasingly used to determine femtosecond and attosecond charge transfer processes initiated at an atomically defined starting point. We give proof that the time constant of a charge transfer process determined by the core-hole-clock method is probed, but not governed by the spectroscopic process. To this end, charge transfer in c(4 x 2)S/Ru(0 0 0 1) has been probed by two investigations using core level resonances with identical symmetry but excitation photon energies one order of magnitude apart (S2s -> 3p at 227.5 eV, S s -> 3p at 2471.1 +/- 0.3 eV). Despite the large difference in excitation energy, the measured charge transfer times agree. (c) 2006 Elsevier B.V. All rights reserved.