A description is given of the use of asymmetrically substituted urea derivatives of the general formula (I) where R1 and R2 each independently are a linear or branched aliphatic hydrocarbon radical having 1 to 4 carbon atoms as accelerators in combination with dicyandiamide as latent hardener for epoxy resin systems based on unhalogenated or halogenated bisphenols of type A or F and also based on resorcinol or tetrakisphenylolethane. The advantages of the inventively proposed accelerator/hardener combination are, for example, the excellent reactivity and very good storage stability. Moreover, the mechanical properties of the resins cured accordingly are likewise outstanding.