Even before weight loss in the low-temperature pyrolysis of cellulose becomes significant, the average degree of polymerization of the partially pyrolyzed samples drops sharply. The gel permeation chromatograms of nitrated derivatives of the samples can be described in terms of a small number of mixed size populations—each component fitted within reasonable limits by a Weibull distribution function. The modal value of each component is taken as an integral multiple of a “macromonomer” of nominal degree of polymerization 228.4. Thus, the data are consistent with a model in which the degradation process is assumed to involve bond rupture at specific “weak links” into particularly favored molecular sizes. Such a mechanism provides a plausible alternative to the commonly assumed random breakdown along the length of the molecule.