In this study, the cationisation of galactomannan and xylan hemicellulose types was investigated with 2,3-epoxypropyltrimethylammonium chloride (ETA) as reagent, under alkaline conditions. The degree of substitution was accurately determined by H-1 NMR spectroscopy after hydrolysis in a DCl/D2O mixture and confirmed by elemental analyses. The influence of reactional parameters on yield and DS was studied systematically and the reactivity of both polysaccharides was compared. DS values from 0.1 to 1.3 were obtained by varying reactional parameters, the most influential being the hemicellulose concentration and the epoxide amount. The corresponding mass yields varied from 40 to 80% of recovered hemicelluloses and were strongly correlated to DS values. The cationisation of xylan was studied more particularly with a D-optimal experimental design in order to identify interactions between parameters. The optimization of reactional parameters as a function of a desired DS allowed obtaining a maximal grafting rate of 48% with a DS value of 1.3 and 64% of mass yield. (C) 2011 Elsevier Ltd. All rights reserved.