Cellulose and starch derivatives are often prepared by substitution reactions to the hydroxyl groups(OH-2, OH-3 and OH-6) of the corresponding an hydroglucose units. A general kinetic model for the substitution of cellulose and starch was developed. The model assumes different reactivities of the hydroxyl groups and a decrease in the reactivity as the substitution proceeds. Both phenomena are taken into account in the model. The model predicts the detailed distribution of the different mono (2, 3, 6)-, di (23, 26, 36)- and trisubstituted (236) units as a function of the reaction time. The classical Spurlin distribution is obtained as a special case of the general model. Numerical strategies were developed for the solution and computer simulation of the model. Simulation results were shown to be valid for various reactivity ratios of the hydroxyl groups. The modelling concept was verified with experimental data obtained for carboxymethylation of cellulose. It should be emphasized, however, that the same general approach is also applicable in studies of any hemicelluloses or oligo-or monosaccharides. The detailed data, which revealed the non-uniform reactivities of the different hydroxyl groups and the decline of the substitution rate with time, were successfully described by the mathematical model. (C) 2010 Elsevier Ltd. All rights reserved.