Secondary diamine N,N′-dialkyl methylcyclohexanediamine acts as a reactive diluent for curable epoxy resin compositions. The addition of this compounds significantly reduces the initial viscosity of the epoxy resin composition while the resulting cured epoxy resin exhibits comparable favorable mechanical, chemical resistance and thermal properties such as low water uptake and high glass transition temperatures. Such compositions are particular suitable for manufacturing of composites with high mechanical and heat resistance properties by the means of resin transfer molding (RTM), vacuum aided resin transfer molding (VARTM) or infusion technology.