Crystallization from a 20-year-old commercial source of 3-ethyl-4-methyl-3-pyrrolin-2-one afforded 1:1 co-crystals of this compound (C7H11NO) with its oxidized derivative, 3-ethyl-4-methyl-3-pyrroline-2,5-dione (C7H9NO2). The compound crystallizes in the space group P\overline{1}, with two molecules of each species in the asymmetric unit. These four molecules form a hydrogen-bonded tetramer with a dimer of 3-ethyl-4-methyl-3-pyrrolin-2-one as the core flanked by one molecule of the dione on each side.
