Glycerol conversion was conducted in hot-compressed water (HCW: 573-673 K, 25-34.5 MPa) using a batch and a flow apparatus and the influences of temperature, H2SO4, glycerol concentration, and pressure, were examined. The yield of acrolein was enhanced by higher glycerol and H2SO4 concentration, and higher pressure. Approximately 80% selectivity of acrolein was obtained at 90% of glycerol conversion with an acid catalyst in supercritical condition (673 K and 34.5 MPa). The rate constant of acrolein decomposition was always higher than that of acrolein formation in the absence of acid catalyst but the rate constant of acrolein formation could be overcome that of acrolein decomposition by addition acid in supercritical condition. (c) 2006 Elsevier Ltd. All rights reserved.