A method includes mixing a clathrate compound with an epoxy resin to form a curable epoxy resin composition, and may further include heating the curable epoxy resin composition to cure the curable epoxy resin composition. The clathrate compound includes (a1) a carboxylic acid compound represented by the formula A(COOH).sub.k; and (a2) a xylylene diamine compound. In the formula A(COOH).sub.k, A represents a C1-C6 linear hydrocarbon group optionally having a substituent, a C3-C10 monocyclic hydrocarbon group optionally having a substituent, or a C6-C10 bicyclic hydrocarbon group optionally having a substituent; and k represents 2 or 3.

In general, the present invention relates to lignin-derived curing agents, as well as their use in resin compositions. Furthermore, the present invention relates to methods for preparing said lignin-derived curing agents.