The microwave-assisted hydrothermal treatment of fructose and glucose is studied under subcritical conditions, in a temperature window between 180 degrees C and 250 degrees C. The suitability of microwave heating as well as the impact of the reaction variables temperature and time on the extent of the substrate conversion and on the product composition are elucidated. With rate constants of 3.6.10(-3) s(-1) (fructose) and 5.2.10(-4) s (-1) (glucose) at 220 degrees C the decomposition of fructose took place about eight times faster than that of glucose. 5-HMF was found to be the major decomposition product for both monosaccharides, with maximum yields of 47% 5-HMF (for fructose) and 30% (for glucose) at a maximum selectivity of 65% and 50%, respectively. All other liquid monomeric products like levulinic acid, furfural and trihydroxy benzene are formed at cumulative yields below 16%. (C) 2012 Elsevier Ltd. All rights reserved.