[en] The cause of the time scale for ultrafast demagnetization induced by irradiation using a fs pulse laser remains an open issue. Spin-flip mediated by electron–phonon scattering due to spin–orbit interactions is one major theory proposed to explain ultrafast demagnetization. Ultrafast demagnetization in Ni, FePd, and FePt films was investigated in order to systematically study the influence of heavy elements on the demagnetization time. The ultrafast demagnetization in these systems was analyzed using the microscopic three temperature model, which is a theory based on spin-flip mediated by electron–phonon scattering. The spin-flip probability ($a_{\text{sf}}$) values for the Ni, FePd, and FePt films were evaluated in the low pump fluence regime. It was found that the $a_{\text{sf}}$ value for the Ni film was larger than that for the FePd and FePt films. Thus, there is no correlation between the $a_{\text{sf}}$ value and the spin–orbit coupling strength. Fast demagnetization of the FePd film was also observed due to large electron–phonon scattering. In addition, it was found that the $a_{\text{sf}}$ values decreased with increasing magnetization quenching for all the films. (paper)