Cellulose fibrils (F1) with a length of 1 micron to 2 mm, diameter of 0.005-10 microns and aspect ratio of 200-10000 are claimed. Also claimed are the following : (i) the use of natural or regenerated cellulose fibres (F2) for the preparation of these fibrils; (ii) the preparation of (F1) by mechanical treatment of (F2) ; (iii) the preparation of cellulose fibre fibrils using ultrasonic energy ; (iv) the preparation of cellulose fibre fibrils by vapour flashing ; (v) cellulose products (P1) containing more than 500 (F1) per mg ; (vi) cellulose products (P2) with specific surface areas of at least 0.75 m<2>/g containing (F1); and (vii) mixtures (M) of (F1) and (F2) containing at least 20% (F1).
