Organosolv fractionation allows the separation of lignocellulosic biomass in its three major components; sugars from hemicelluloses, cellulosic fibres (pulp), and solid, low molecular weight, lignin. The kinetics of fractionation of wheat straw in aqueous ethanol was studied and a new generalised kinetic model was proposed to simulate the experimental results. This model was applied to correlate the kinetic results for non-isothermal experimental conditions, incorporating the effects of reaction time, temperature and acid concentration. Using the proposed model, optimal fractionation time, in terms of the quality of the resulting cellulose fraction, can be calculated for the conditions of a given experiment. As an example, for isothermal reaction conditions at 140 degreesC and catalyst concentration equal to 0.2 N H2SO4, Minimum lignin content (2% w/w on the resulting pulp) would be obtained for 52 min reaction time. At these conditions, pulp yield 42% (w/w on dry straw), maximum solubilized lignin 96% (w/w on original lignin), and soluble total sugars 27% (w/w on the original straw polysaccharides) would be obtained. (C) 2003 Elsevier Ltd. All rights reserved.