An autoclave (120-mL) and an optical micro-reactor (50-nL) were used to study the hydrothermal decomposition of D-glucose at high temperatures and high pressures. During slow heating (0.18 degrees C/s) to 350 degrees C in the autoclave, water-soluble glucose (0.9 M) began to decompose at 220 degrees C and reacted completely at 280 degrees C. The initial decomposition products were 5-(hydroxymethyl)furfural and levoglucosan, and these subsequently converted into oil and solid residue, and finally to solid particles at a 65 wt% yield at 350 degrees C. When the same heating rate and temperature were used on glucose solutions in the micro-reactor, yellow and orange materials decomposed from glucose were produced. Numerous particles precipitated at 251 degrees C, and at 350 degrees C, all the glucose changed to an orange film and solid particles, which were nanopartides as confirmed by SEM. However, when the glucose solution was rapidly heated to 410 degrees C (9.5-17 degrees C/s), yellow, brown and orange sugar-like materials were produced. A homogeneous phase with yellow color still remained at temperatures as high as 380 degrees C, and few particles formed until 410 degrees C. It can be concluded that micron-sized particles and colored solutions can be produced by slow heating, while rapid heating resulted in the formation of dye-like substances with glucose-like structures. The formation of colored solutions and particles may have technological implications in food or materials formation processes that use high temperature water with biomass feedstocks. (C) 2010 Elsevier B.V. All rights reserved.