Direct observations of the heating of microcrystalline cellulose (230 DP) in water at temperatures up to 410°C and at pressures up to 700 MPa were made with a batch-type microreactor. Cellulose particles were found to dissolve with water over temperatures ranging from 315 to 355°C at high pressures. Dissolution temperatures depended on water density and decreased from about 350°C at a water density of 560 kg/m 3 to a minimum of around 320°C at a water density of 850 kg/m 3 . At densities greater than 850 kg/m 3 , the dissolution temperatures increased and reached a value of about 347°C at 980 kg/m 3 . The cellulose dissolution temperatures were independent of heating rates for values ranging from 10 to 17°C/s. The low dependence of dissolution temperatures on the heating rates is strong evidence for simultaneous dissolution and reaction of the cellulose. Different phenomena occurred depending on water density. At low densities, particles turned transparent and seemed to dissolve into the aqueous phase from the surface. From 670 to 850 kg/m 3 , the cellulose particles visibly swelled just before completely collapsing and dissolving into the aqueous phase. The swelling probably increased water accessibility and particle surface area and thus lead to the lower dissolution temperatures observed. From 850 to 1000 kg/m 3 , the particles required longer times to dissolve and many fine brown-like particles were generated as the particles dissolved. FT-IR spectra of the residues were analyzed. Residues formed from heating cellulose at high densities still retained some cellulose character whereas those as low densities had little cellulose character, especially in the O -H stretching vibration region.

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