AbstractA discussion of the properties of an “ideal” cellulose fibre consisting of a cellulose single crystal reveals, that there is not only a large discrepancy between the theoretical tensile strength of such a fibre and the highest experimentally observed strength of actual fibres, (as already pointed out by Meyer, Mark and de Boer) but that also the ideal fibre would have an extensibility of more than 40 %, whereas the actual elongation at rupture of well oriented cellulose fibres is very much lower. From a calculation of the amount of energy necessary to bring about the rupture of the ideal fibre, it follows, that this fibre would explode after rupture.It appears therefore, that the ideal fibre as hitherto postulated, is to be regarded as a mere theoretical abstraction, which has little if any bearing on the problem of the actual mechanism of rupture.There is no certain evidence for the assumption made by previous authors, that in the actual process of rupture only cohesional attractions between primary valence chains are overcome and that there would be no rupture of the chains themselves.Irrespective as to whether in the real process of rupture primary valencies are actually broken or not, it can be shown, that the energy involved in this process, ranging between 2.108and 6.108erg/cm3, would be sufficient to bring about a considerable drop in the average chain length of the molecules in the fibre. If the energy of rupture is to any appreciable percentage connected with the rupture of molecular chains, the effect should be easily detectable by viscosity determinations. Experiments with the purpose to investigate if this effect actually exists will be undertaken.New experimental data on the energy at rupture of artificial cellulose fibres of different degrees of orientation (ranging from isotropic to extremely well oriented ones) and of commercial viscose rayon filaments, are gi