The invention relates to a security element (1), comprising a first luminescent substance, for a security document (5), the latter (5) comprising a base body (51) consisting of a base material. The aim of the invention is to increase the protection against forgery. To achieve this: the security element is provided with a second luminescent substance; energy is transmitted between the first luminescent substance and the second luminescent substance, whereby the frequency range of the excitation of one of the luminescent substances corresponds to the emission frequency range of the other luminescent substance; and the first luminescent substance is thermally unstable at the ignition temperature of the base material and/or at a temperature of 185° C.