carbanilation reactions of cellulose samples (bleached cotton linters and avicel) with phenylisocyanate in dimethylsulfoxide (dmso) were carried out at 60 $degree$ c in the presence of various pyridine derivatives. the molecular weight distributions of the resulting cellulose tricarbanilates (ctcs) were measured by high-performance size exclusion chromatography. when pyridine or its derivatives were included in the reactions, the ctcs had reduced degree of polymerization (dp) values compared to those of ctcs prepared without the additives. the cellulose depolymerization was promoted by pyridines with electron-donating substituents and was not favored by pyridines with electron-withdrawing substituents nor with groups at positions ortho to the pyridine ring nitrogen atom. for the 3-, 4-, and 3,4-substituted pyridines, there was a linear relationship between log (weight-average ctc dp) and the pk//a (in water) of the pyridine derivative. for 2- and 2,6-substituted pyridines, the dp-pk//a relationships were different, probably because of steric effects of the different substituents ortho to the pyridine nitrogen atom. the optimum dmso: pyridine solvent ratio for cellulose depolymerization during carbanilation in dmso: pyridine mixtures was 3:1. all three components, phenylisocyanate, pyridine or its derivatives, and dmso, are required for the depolymerization reaction. it is suggested that the depolymerization may be a consequence of cellulose oxidation by dmso and/or cleavage of glucosidic bonds of partially carbanilated celluloses by reactions promoted by an enhanced solvent