The invention relates to a curing agent for thermosetting epoxy resins comprising (a) at least one aromatic amine containing at least two amino groups, and (b) at least one clathrate compound obtained by reacting a tetrakisphenol of the formula (1), or a 9,9-Bis(4-hydroxyphenyl)fluorene of formula (2), as the host molecule, and an imidazole or an imidazolium derivative as the guest molecule, wherein the substituents are as defined in the description, and n is the number 0, 1, 2 or 3, which can advantageously be used for the curing of epoxy resins. In addition, the invention also relates to a process for the preparation of a cured article, a process for the preparation of insulation systems for electrical engineering and a cured article obtained by the processes.