This paper describes a study by infrared, deuteration, and other techniques of the fine structure of three regenerated cellulose fibers (Fortisan, a super tire yarn, and a fiber of high wet modulus). The infrared and deuteration measurement provide information on the amount and perfection of the hydrogen-bond ordered material in these celluloses. The three fibers are markedly different in structure: the Fortisan contains about 40% ordered cellulose of a high average degree of perfection, whereas the super tire yarn contains a smaller amount (about 20%) of ordered cellulose of a lower average degree of perfection: the yarn of high wet modulus contains about as much ordered material as the Fortisan, but of an average degree of perfection more similar to that of the super tire yarn. Infrared-dichroism measurements on the fibers are described. The infrared and dichroism studies are discussed in relation to the results of x-ray, moisture regain, acid hydrolysis, and birefringence measurements on the fibers. The infrared-deuteration behavior of acid hydrolysis residues prepared from the three fibers is described; these residues are more highly ordered than the patent fibers, but yield no information of any value in characterizing the structures of the parent fibers.