Experimental and reported data on the cleavage of acetyl groups and hemicellulose removal during hydrothermal treatments of several lignocellulosic raw materials were interpreted using a generalised calculation procedure based on the severity factor. In the mathematical equations describing the hydrolysis of xylan (the main hemicellulosic polymer) and acetyl groups, the correction of the potential concentrations of both fractions to consider the presence of non-reactive portions of the substrates led to an improved reproduction of the experimental data. The generalised analysis of data allowed the calculation of activation energies for the two reactions considered, providing a fair interpretation of results obtained under a variety of experimental conditions. (C) 2002 Elsevier Science Ltd. All rights reserved.