Cellulose nanocrystals (CNC) exhibit potentially good mechanical properties which make them interesting as candidates for reinforcing polymers, i. e. for manufacturing of nanocomposite materials. Incorporation of such elements into a polymer matrix requires however a careful characterization of the particles, their effect on the rheological behaviour of different types of dispersions/suspensions and on the affinity towards matrix material. In case of the last point, a surface treatment of the CNC may be required in order to obtain a sufficiently strong interphase region. In the present work, CNC dispersions were prepared from microcrystalline cellulose by a sulfuric acid treatment. The produced CNC were subsequently surface modified or functionalized with three different azetidinium salts. The surface treatment substantially improved the thermal stability of the CNC (which initially contained thermally instable sulfate groups) as revealed by thermogravimetric analysis. The rheological behaviour of aqueous dispersions containing 0.65 and 1.3 weight-% of the modified CNC was evaluated at room temperature. The surface treatment of the CNC increased significantly the shear viscosity as well as the dynamic (storage and loss) moduli of the dispersions (the extent depending of the modifying agent used). Furthermore, the functionalization reduced the gel point of the CNC dispersion from 2.5 weight-% (unmodified state) down to 02-0.4 weight-%. The implications of the surface treatment with regard to rheological behaviour and use in composite materials will be further commented on.