The widely accepted Broido-Shafizadeh model describes cellulose pyrolysis kinetics in terms of two parallel (competing) reactions preceded by an initiation step. In spite of the fact that many recent experimental results seem to contradict the predictions of the model, its validity has not been seriously questioned. In this paper we report thermogravimetric analyses of Avicel cellulose involving prolonged thermal pretreatments of small samples (0.5-3 mg). The weight loss curves were simulated by modern numerical techniques using the Broido-Safizadeh and other related models. Results were not consistent with the presence of an initiation reaction, but they did strongly confirm the role of parallel reactions in the decomposition chemistry. A subsequent, high-temperature (370 degrees C), pyrolytic degradation of solid intermediates formed below 300 degrees C was also detected. In the absence of a prolonged thermal pretreatment, only one of the two parallel reactions can be observed. This reaction is first order, irreversible, and manifests a high activation energy (238 kJ/mol). The kinetic parameters of this reaction are not influenced by the large quantity of solid intermediates formed during prolonged, low-temperature thermal pretreatments, indicating that chemical processes are much more significant than the physical structure of the sample during pyrolysis.