It has long been known that cellulosic materials exhibit acidic properties which influence many of their physical and chemical characteristics. Thus, the acidic groups of the fibre bind small amounts of ash which have a considerable effect on the electrical conductivity of cotton fibres, and on the viscosities of solutions of some cellulose derivatives. The ash may also affect the dyeing properties of the fibre, and may be related to the application of finishing agents. Since the ash of washed cotton consists prin cipally of cations held by acidic groups, the problems of measuring the ash and the number of acidic groups on the fibre are intimately related. In addition, if some of the acidic groups of purified cot ton are a part of the cellulose molecule, such as end-groups, the number of these groups may be a measure of its chain length, which is of primary importance in determining the strength and other physical characteristics of the fibre.As part of a general program for the investigation of the acidic and basic properties of textile fibres, a study has been made of the manner in which hydrochloric acid is bound by cotton, and of the dependence of the amount of acid bound on the concentration of the aoid solution in contact with the fibre. The effect on this dependence of different amounts of an added neutral salt, potas sium chloride, has also been studied.A comparison of the acid bound with the cationic ash (the total content of the cations of inorganic bases) as determined by an electrodialysis method leads to the conclusion that the maximum acid-binding capacity is equivalent to the cationic ash, and that therefore the acid bound at any acidity depends upon the ash. A comparison of the results for dewaxed cotton with those for de pectinized cotton shows that most of the acidic groups of cotton are contributed by the pectic substance.Cotton freed of pectic substance retains a small number of acidic groups. Although other possibilities exist, it seems likely that these are an integral part of the cellulose molecule, such as an end-group. If this is actually the case, the results show that the cotton cellulose ~rnolecule has an equ.ivatent weight of about 100,000 which corresponds to a minimum chain length of about 600 glucose residues.