FORMALDEltYDE and its derivatives are reagents which are widely used in reedylying cellulose materials. Numerous findings about the mechanism of the reaction of formaldehyde with cellulose are reported in the literature, about the part played by catalysts and the modification methods [1-4], bu t there is almost nothing about the supermolecular changes of the cellulose structure on treating it with formaldehyde. Some of the reports by Tripp and Rollins [5, 6] indicate that there is a relationship between the solubility of cotton fibre cuttings treated with formaldehyde and the weight increase due to modification. Numerous investigators believe that a decisive part is played by the structural changes and the resulting formation of transverse bonds between cellulose macromolecules in the observed property changes of formaldehyde-modified cellulose (improved crease resistance and brittleness of the fibres, lower irreversible deformation, etc.). The s tudy of the modification of cellulose with formaldehyde is also of interest with respect to the crosslinking which takes place without formation of a homopolymer network, normally mal~ng structural investigations of modified cellulose difficult. Finally, the few structural investigations which we mentioned were made with cotton cellulose samples, and clarification of the supermolecular structural changes occurring during the formaldehyde treatment of cellulose preparations with different structural characteristics would be interesting. We examined the supermolecular structure of a series of such formaldehydemodified cellulose preparations by various electron-microscopic methods. In addition, some physicomechanical properties of the samples, such as their strength, elongation, crease resistance, and also the elongation curves of fibres in alkali, were determined.