A method for purifying N-(2-hydroxyethyl)-2-pyrrolidone (HEP) is disclosed. The method comprises crystallizing crude HEP to produce HEP crystals and a mother liquor, and separating the HEP crystals from the mother liquor. In one method of the invention, crystallization is induced by adding an HEP seed crystal to the crude HEP. In a preferred method, the crystallization is performed in the presence of 1-4 wt. % of added water. HEP can be successfully crystallized to a purity greater than 99.9%.