Resonant photoemission spectroscopy has been used to investigate the spin-dependent electron transport from core-excited argon monolayers to ferromagnetic substrates with subfemtosecond precision. Charge transfer times as a function of the spin of the resonantly excited electron and the magnetization of the substrate have been studied for Ar=Feð110Þ, Ar=Coð0001Þ, and Ar=Nið111Þ. For argon adsorbed on iron and cobalt, a significantly faster charge transfer is found for minority electrons, whereas no such spin dependence is obtained for nickel. The results are explained considering the different empty densities of states of the metals for minority and majority electrons.