In the same thermogravimetric analyzer (TGA) under identical conditions, samples of pure, ash-free cellulose (i.e., Avicel PH-105, Whatman CF-11, Millipore ash-free filter pulp, and Whatman #42) obtained from different manufacturers undergo pyrolysis at temperatures which differ by as much as 30 degrees C. Thus, the pyrolysis chemistry of a sample of pure cellulose is not governed by a universal rate law, as is the case with a pure hydrocarbon gas (for example). Nevertheless, the pyrolytic weight loss of all the samples studied in this work is well represented by a high activation energy (228 kJ/mol), first-order rate law at both low and high heating rates. These results do not corroborate the recent findings of Milosavljevic and Suuberg (Ind. Eng. Chem. Res. 1995, 34, 1081-1091). For a particular cellulose sample (for example, Avicel PH-105), variations in the preexponential constant determined at different heating rates reflect uncontrolled, systematic errors in the dynamic sample temperature measurement (thermal lag).