Abstract: We describe an experiment in which the magnetism of a ferromagnetic thin-film is modified with an external electric field. The magnetocrystalline anisotropy of ordered FePt (FePd) compounds which have perpendicular magnetocrystalline anisotropy can be modified by an applied electric field when immersed in an electrolyte. The modification of the magnetic parameters is attributed to a change in the number of unpaired d electrons in response to the large electric field because of the formation of an electrolytic double layer in front of the ferromagnetic thin-film. The experimental results demonstrated that a voltage change of -0.6 volts on a 2 nm thick film altered the coercivity by -4.5% and﹢1% in FePt and FePd, respectively. This is the first experimental observation of the modification of the magnetism in thin-film ferromagnets by an electric field.