Cellulose-starch gel mixtures (4 wt% cellulose and 4 wt% starch gel) were mixed with water in a 9: 1, water:organic, volume ratios and rapidly heated (ca. 20 s) to high-temperatures (ca. 520 degrees C) and high-pressures (ca. 800 MPa) in 0.04 mu L microreactors to examine their characteristics and reaction products. Contents of the microreactors were observed during the heating with microscopy and residues were analyzed with chromatography and spectroscopy. At high water loading densities (ca. 980 kg/m(3)), heating of either starch gels or cellulose-starch gel mixtures gave a light yellow colored liquid associated with 5-hydroxymethylfurfural along with solid products that had strong absorptions at 1630 and 15 30 cm(-1) associated with aromatic and polycyclic ring compounds. At low water loading densities (<700 kg/m(3)), a brown colored liquid was generated that had an oil-like, paraffinic hydrocarbon character along with gases, but no particles were formed. The cellulose-starch gels studied in this work can possibly be used as feedstocks in continuous batch microreactor systems. (C) 2007 Elsevier Ltd. All rights reserved.