In 0.1 M sulfuric acid at 125 degrees C, levulinic acid did not form humins, and if HMF was present, levulinic acid was not incorporated in the humins that formed from it. Levulinic acid was converted to unidentified products believed to include angelica lactones, at a rate more than 500 times smaller than the rate of HMF conversion. Quantum chemical calculations indicate that at reaction conditions, levulinic acid is protonated to form protonated dihydro-5-hydroxy-5-methyl-2(3H)-furanone which is very stable. Very small amounts of levulinic acid and 5-hydroxy-gamma-valerolactone may be present in equilibrium with this cation. Angelica lactones can then be formed from 5-hydroxy-gamma-valerolactone in an acidic aqueous environment. (C) 2015 Elsevier B.V. All rights reserved.