NaOH catalyzed condensation reaction between 5-hydroxymethylfurfural and levulinic acid in water gives a 2.5: 1 mixture of aldol products: (E)-6-[5-(hydroxymethyl)furan-2-yl]-hex-4-oxo-5-enoic acid and (E)-3-[5-(hydroxymethyl)furan-2-yl]methylene-4-oxo-pentanoic acid in 82% combined yield. A similar reaction between furfural and levulinic acid gives the linear polymer, poly[1-carboxymethyl-4-(furan-2-yl)-2-oxo-butane-1,4-diyl] in 91% yield.